Metal mediated ammonia production and systems for performing the same

Abstract

Described herein are electrochemical methods for producing ammonia. In one aspect, the method involves introducing nitrogen gas at a pressure greater than ambient pressure into an electrochemical cell composed of an electrolyte composition and applying a current to the electrochemical cell to convert nitrogen to ammonia. In one aspect, the electrolyte composition includes (i) an organic solvent, a (ii) a proton donor, and (iii) a salt of lithium, calcium, magnesium, strontium, yttrium, scandium, zirconium, or any combination thereof. The methods described herein provide increased ammonia Faradaic efficiency (FE) while using less energy (i.e., applied current).

Description

METAL MEDIATED AMMONIA PRODUCTION AND SYSTEMS FOR PERFORMING THESAMECROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of and priority to co-pending U.S. Provisional Patent Application No. 63 / 537,336, filed on September 8, 2023, and 63 / 602,293, filed on November 22, 2023, the contents of which are incorporated by reference herein in their entireties.BACKGROUND

[0002] Ammonia plays a crucial role in agriculture as a fertilizer, and its potential as a carrier of renewable energy is increasingly recognized due to its high hydrogen content and energy density. The current method for producing ammonia, namely the Haber-Bosch process, is characterized by its high energy demand, requiring extremely high temperatures and pressures ranging from 350°C to 450°C and 100 to 200 bar, respectively. Moreover, this conventional process, reliant on fossil fuels, is economically feasible only within large, centralized plants. However, as the cost of renewable electricity declines, there has been growing interest in electrochemical approaches for ammonia production. These methods offer the potential for distributed production from intermittent renewable energy sources, with the added benefits of zero CO2 emissions and lower capital costs.

[0003] While numerous nitrogen reduction systems in aqueous media, featuring various configurations and catalyst compositions, have been proposed, many suffer from low Faradaic efficiencies (FEs) and production rates due to competing HER, rendering them impractical for widespread use. Additionally, the solubility of nitrogen is less in aqueous solution compared to organic solutions. Furthermore, systems with very low FE and partial current density face challenges related to ammonia contamination, making their reproducibility more difficult.SUMMARY

[0004] Described herein are electrochemical methods for producing ammonia. In one aspect, the method involves introducing nitrogen gas at a pressure greater than ambient pressure into an electrochemical cell composed of an electrolyte composition and applying a current to the electrochemical cell to convert nitrogen to ammonia. In one aspect, the electrolyte composition includes (i) an organic solvent, a (ii) a proton donor, and (Hi) a salt of lithium, calcium, magnesium, strontium, yttrium, scandium, zirconium, or any combinationthereof. The methods described herein provide increased ammonia Faradaic efficiency (FE) while using less energy (i.e. , applied current).

[0005] Other systems, methods, features, and advantages of the present disclosure will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present disclosure, and be protected by the accompanying claims. In addition, all optional and preferred features and modifications of the described embodiments are usable in all aspects of the disclosure taught herein. Furthermore, the individual features of the dependent claims, as well as all optional and preferred features and modifications of the described embodiments are combinable and interchangeable with one another.BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Many aspects of the present disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.

[0007] FIGS. 1 A-1 B show benchmarking experiments. (A) NH3 FE as a function of the applied current densities. (B) NH3CD as a function of the applied current densities\

[0008] FIGS. 2A-2B show (A) NH3FE as a function of pressure and (B) NH3CD as a function of pressure. The current density is switched between -150 mA / cm2and 0 mA / cm2for a period of 2 min each. 2M I CIO4 is dissolved in THF containing 0.065 M EtOH.

[0009] FIGS. 3A-3F show the effect of proton donor concentration (A) NH3FE as a function of the EtOH concentration at 20 bar. (B) NH3CD as a function of the EtOH concentration at 20 bar. (C) NH3FE as a function of the EtOH concentration at 50 bar. (D) NH3CD as a function of the EtOH concentration at 50 bar. (E) NH3FE as a function of the EtOH concentration at 100 bar. (F) NH3CD as a function of the EtOH concentration at 100 bar. The current density is switched between -150 mA / cm2and 0 mA / cm2for a period of 2 min each. 2M l_iCIO4is dissolved in THF containing EtOH.

[0010] FIGS. 4A-4B show the effect of alkyl chain length of proton donors. (A) NH3FE as a function of different proton donors. (B) NH3CD as a function of different proton donors. The current density is switched between -150 mA / cm2and 0 mA / cm2for a period of 2 min each. 2M IJCIO4 is dissolved in THF containing 0.1 M of proton donor. N2pressure is set to 20 bar.

[0011] FIGS. 5A-5B show the effect of Li salt concentration. (A) NH3FE as a function of the LiBF4concentration. (B) NH3CD as a function of the LiBF4concentration. The current density is switched between -150 mA / cm2and 0 mA / cm2for a period of 2 min each. LiBF4is dissolved in THF containing 0.065M of EtOH. N2 pressure is set to 20 bar.

[0012] FIGS. 6A-6D show different efficiencies (EE - Energy Efficiency, VE - Voltage Efficiency, and FE - Faradaic Efficiency). (A) Efficiencies as a function of the applied current densities. (B) Efficiencies as a function of pressure. (C) Efficiencies as a function of the EtOH concentration at 20 bars. (D) Efficiencies as a function of the LiBF4concentration.

[0013] FIG. 7 shows the effect of proton source on voltage efficiency. Voltage efficiency versus cell potential for LiMAS with various proton sources such as H2O, H2and EtOH.

[0014] FIGS. 8A-8B show CapEx and OpEx estimates. (A) Total CapEx cost of LiMAS plant for different NH3current densities of the high-pressure electrochemical cell. The CapEx estimate includes the cost of high-pressure autoclave-based electrolyzers and ancillary equipment such as air separators, compressors, heat exchangers, transformers, chillers etc. (B) Total OpEx cost of LiMAS plant as a function of cell voltage of the electrochemical cell. The OpEx cost includes electricity consumption of electrochemical cells and utilities to operate ancillary equipment.

[0015] FIGS. 9A-9D show (A) schematic and configuration of the batch autoclave for electrochemical ammonia synthesis; (B) NH3FE and NH3current density (CD) at different applied CDs; (C)1H-NMR spectra for14N experiments at varying current densities; and (D)1H- NMR spectra for15N experiment at -15 mA / cm2.

[0016] FIGS. 10A-10D show (A) SEM image of Ca deposited Ni foam post electrolysis; (B) EDS spectrum of post electrolysis electrode showing the presence of Ca and Ni; (C) high resolution XPS scan of the post electrolysis electrode confirming the presence of Ca; and (D) XPS Survey scan of the post electrolysis electrode showing presence of O from perchlorate species.

[0017] FIGS. 11A-1 1 B show representative chronopotentiometry. (A) Applied current density, total cell potential and charge as a function of time. Current density was switched between - 30 mA / cm2and 0 mA / cm2, the total charge passed was -1 10 C. B) Zoomed version clearly denoting the applied current density, cell potential, and charge.

[0018] FIGS. 12A-12B show (A) NH3CD at different applied CDs and (B) NH3FE at different applied CDs

[0019] FIGS. 13A-13B show a comparison of NH3FE and NH3CD for LiMAS, MgMAS and CaMAS.

[0020] FIGS. 14A-14B show (A) post and pre electrolyte NMR spectra for control experiment under 6 bar Ar and (B) NMR spectra after keeping freshly prepared electrolyte open in the fume hood for 2 hours NMR, post and pre electrolyte spectra for open circuit control experiment.

[0021] FIGS. 15A-15B show (A)1H-NMR spectra for experiments at varying current densities and (B) NMR calibration curve with various current density points.

[0022] FIGS. 16A-16D show (A) SEM image of Mg deposited Ni foam post electrolysis; (B) EDS spectrum of post electrolysis electrode showing the presence of Mg and Ni; (C) high resolution XPS scan of the post-electrolysis electrode confirming the presence of Mg° (1s); and (D) high resolution XPS scan of the post-electrolysis electrode confirming the presence of Mg° (2p).

[0023] FIG. 17 shows an exemplary electrochemical cell described herein.

[0024] FIG. 18 shows an exemplary system for continuously producing ammonia using the methods described herein.

[0025] Additional advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or can be learned by practice of the invention. The advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.DETAILED DESCRIPTION

[0026] Many modifications and other embodiments disclosed herein will come to mind to one skilled in the art to which the disclosed compositions and methods pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the disclosures are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. The skilled artisan will recognize many variants and adaptations of the aspects described herein. These variants and adaptations are intended to be included in the teachings of this disclosure and to be encompassed by the claims herein.

[0027] Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

[0028] As will be apparent to those of skill in the art upon reading this disclosure, each of the individual embodiments described and illustrated herein has discrete components and features which may be readily separated from or combined with the features of any of the other several embodiments without departing from the scope or spirit of the present disclosure.

[0029] Any recited method can be carried out in the order of events recited or in any other order that is logically possible. That is, unless otherwise expressly stated, it is in no way intended that any method or aspect set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not specifically state in the claims or descriptions that the steps are to be limited to a specific order, it is no way intended that an order be inferred, in any respect. This holds for any possible non-express basis for interpretation, including matters of logic with respect to arrangement of steps or operational flow, plain meaning derived from grammatical organization or punctuation, or the number or type of aspects described in the specification.

[0030] All publications mentioned herein are incorporated herein by reference to disclose and describe the methods and / or materials in connection with which the publications are cited. The publications discussed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided herein can be different from the actual publication dates, which can require independent confirmation.

[0031] While aspects of the present disclosure can be described and claimed in a particular statutory class, such as the system statutory class, this is for convenience only and one of skill in the art will understand that each aspect of the present disclosure can be described and claimed in any statutory class.

[0032] It is also to be understood that the terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the disclosed compositions and methods belong. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the specification and relevant art and should not be interpreted in an idealized or overly formal sense unless expressly defined herein.

[0033] Prior to describing the various aspects of the present disclosure, the following definitions are provided and should be used unless otherwise indicated. Additional terms may be defined elsewhere in the present disclosure.Definitions

[0034] As used herein, “comprising” is to be interpreted as specifying the presence of the stated features, integers, steps, or components as referred to, but does not preclude the presence or addition of one or more features, integers, steps, or components, or groups thereof. Moreover, each of the terms “by”, “comprising,” “comprises”, “comprised of,” “including,” “includes,” “included,” “involving,” “involves,” “involved," and “such as” are used in their open, non-limiting sense and may be used interchangeably. Further, the term “comprising” is intended to include examples and aspects encompassed by the terms “consisting essentially of’ and “consisting of.” Similarly, the term “consisting essentially of’ is intended to include examples encompassed by the term “consisting of.

[0035] As used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a solvent” include, but are not limited to, mixtures or combinations of two or more such solvents, and the like.

[0036] It should be noted that ratios, concentrations, amounts, and other numerical data can be expressed herein in a range format. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint. It is also understood that there are a number of values disclosed herein, and that each value is also herein disclosed as “about" that particular value in addition to the value itself. For example, if the value “10” is disclosed, then “about 10” is also disclosed. Ranges can be expressed herein as from “about” one particular value, and / or to “about” another particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms a further aspect. For example, if the value “about 10” is disclosed, then “10” is also disclosed.

[0037] When a range is expressed, a further aspect includes from the one particular value and / or to the other particular value. For example, where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the disclosure, e.g. the phrase “x to y” includes the range from ‘x’ to ‘y’ as well as the range greater than x’ and less than ‘y’. The range can also be expressed as an upper limit, e.g. ‘about x, y, z, or less’ and should be interpreted to include the specific ranges of ‘about x’, ‘about y’, and ‘about z’ as well as the ranges of ‘less than x’, less than y’, and ‘less than z’. Likewise, thephrase ‘about x, y, z, or greater’ should be interpreted to include the specific ranges of ‘about x’, ‘about y’, and ‘about z’ as well as the ranges of ‘greater than x’, greater than y’, and ‘greater than z’. In addition, the phrase “about ‘x’ to ‘y’”, where ‘x’ and ‘y’ are numerical values, includes “about ‘x’ to about ‘y’”.

[0038] It is to be understood that such a range format is used for convenience and brevity, and thus, should be interpreted in a flexible manner to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. To illustrate, a numerical range of “about 0.1% to 5%’’ should be interpreted to include not only the explicitly recited values of about 0.1% to about 5%, but also include individual values (e.g., about 1 %, about 2%, about 3%, and about 4%) and the sub-ranges (e.g., about 0.5% to about 1.1 %; about 5% to about 2.4%; about 0.5% to about 3.2%, and about 0.5% to about 4.4%, and other possible sub-ranges) within the indicated range. Thus, for example, if a component is in an amount of about 1%, 2%, 3%, 4%, or 5%, where any value can be a lower and upper endpoint of a range, then any range is contemplated between 1% and 5% (e.g., 1 % to 3%, 2% to 4%, etc.).

[0039] As used herein, the terms “about,” “approximate,” “at or about,” and “substantially” mean that the amount or value in question can be the exact value or a value that provides equivalent results or effects as recited in the claims or taught herein. That is, it is understood that amounts, sizes, formulations, parameters, and other quantities and characteristics are not and need not be exact, but may be approximate and / or larger or smaller, as desired, reflecting tolerances, conversion factors, rounding off, measurement error and the like, and other factors known to those of skill in the art such that equivalent results or effects are obtained. In some circumstances, the value that provides equivalent results or effects cannot be reasonably determined. In such cases, it is generally understood, as used herein, that “about” and “at or about” mean the nominal value indicated ±10% variation unless otherwise indicated or inferred. In general, an amount, size, formulation, parameter or other quantity or characteristic is “about,” “approximate,” or “at or about” whether or not expressly stated to be such. It is understood that where “about,” “approximate,” or “at or about” is used before a quantitative value, the parameter also includes the specific quantitative value itself, unless specifically stated otherwise.

[0040] The term “alkyl group” as used herein is a branched or unbranched saturated hydrocarbon group of 1 to 25 carbon atoms, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, f-butyl, pentyl, hexyl, heptyl, octyl, decyl, tetradecyl, hexadecyl, eicosyl, tetracosyl, and the like. Examples of longer chain alkyl groups include, but are not limited to, a palmitategroup. A “lower alkyl” group is an alkyl group containing from one to six carbon atoms.

[0041] The term “cycloalkyl group” as used herein is a non-aromatic carbon-based ring composed of at least three carbon atoms. Examples of cycloalkyl groups include, but are not limited to, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, etc.

[0042] The term “aryl” as used herein is a group that contains any carbon-based aromatic group including, but not limited to, benzene, naphthalene, phenyl, biphenyl, anthracene, and the like. The aryl group can be substituted or unsubstituted. The aryl group can be substituted with one or more groups including, but not limited to, alkyl, cycloalkyl, alkoxy, alkenyl, cycloalkenyl, alkynyl, cycloalkynyl, aryl, heteroaryl, aldehyde, — NH2, carboxylic acid, ester, ether, halide, hydroxy, ketone, azide, nitro, silyl, sulfo-oxo, or thiol as described herein. The term “biaryl” is a specific type of aryl group and is included in the definition of “aryl.” In addition, the aryl group can be a single ring structure or comprise multiple ring structures that are either fused ring structures or attached via one or more bridging groups such as a carbon-carbon bond. For example, biaryl to two aryl groups that are bound together via a fused ring structure, as in naphthalene, or are attached via one or more carbon-carbon bonds, as in biphenyl. Fused aryl groups including, but not limited to, indene and naphthalene groups are also contemplated.

[0043] The term “salt” as used herein is defined as a dry solid form of a water-soluble compound that possesses cations and anions. When the salt is added to water, the salt dissociates into cations and anions.

[0044] As used herein, a plurality (i.e., more than one) of items, structural elements, compositional elements, and / or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of any such list should be construed as a de facto equivalent of any other member of the same list based solely on its presentation in a common group, without indications to the contrary.

[0045] Concentrations, amounts, and other numerical data may be expressed or presented herein in a range format. It is to be understood that such a range format is used merely for convenience and brevity and thus should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range was explicitly recited. As an example, a numerical range of “about 1 ” to “about 5” should be interpreted to include not only the explicitly recited values of about 1 to about 5, but also to include individual values and sub-ranges within the indicated range. Thus, includedin this numerical range are individual values such as 2, 3, and 4, the sub-ranges such as from 1-3, from 2-4, from 3-5, from about 1 - about 3, from 1 to about 3, from about 1 to 3, etc., as well as 1 , 2, 3, 4, and 5, individually. The same principle applies to ranges reciting only one numerical value as a minimum or maximum. Furthermore, such an interpretation should apply regardless of the breadth or range of the characters being described.

[0046] Disclosed are materials and components that can be used for, can be used in conjunction with, can be used in preparation for, or are products of the disclosed compositions and methods. These and other materials are disclosed herein, and it is understood that when combinations, subsets, interactions, groups, etc., of these materials are disclosed, that while specific reference of each various individual and collective combination and permutation of these compounds may not be explicitly disclosed, each is specifically contemplated and described herein. For example, if a class of molecules A, B, and C are disclosed, as well as a class of molecules D, E, and F, and an example of a combination A + D is disclosed, then even if each is not individually recited, each is individually and collectively contemplated. Thus, in this example, each of the combinations A + E, A + F, B + D, B + E, B + F, C + D, C + E, and C + F, are specifically contemplated and should be considered disclosed from disclosure of A, B, and C; D, E, and F; and the example combination of A + D. Likewise, any subset or combination of these is also specifically contemplated and disclosed. Thus, for example, the sub-group of A + E, B + F, and C + E is specifically contemplated and should be considered disclosed from disclosure of A, B, and C; D, E, and F; and the example combination of A + D. This concept applies to all aspects of this disclosure including, but not limited to, steps in methods of making and using the disclosed compositions. Thus, if there exist a variety of additional steps that can be performed with any specific embodiment or combination of embodiments of the disclosed methods, each such combination is specifically contemplated and should be considered disclosed.Electrochemical Production of Ammonia

[0047] Described herein are electrochemical methods for producing ammonia. Despite significant advancements in aqueous electrochemical ammonia synthesis, Li-mediated NH3synthesis still suffers from poor energy efficiency, notably lower than that of the traditional Haber-Bosch process, primarily due to the highly reducing electroplating potential of Li (~-3.04 V vs SHE). It has been discovered that conducting the electrochemical production of ammonia under a positive pressure of nitrogen increases ammonia Faradaic efficiency (FE) while using less energy (i.e., applied current).

[0048] In one aspect, the method involves introducing nitrogen gas at a pressure greater than ambient pressure into an electrochemical cell composed of an electrolyte composition andapplying a current to the electrochemical cell to convert nitrogen to ammonia. An exemplary electrochemical cell is depicted in Figure 17. Referring to Figure 17, the electrochemical cell 1 includes a cathode 2 and an anode 3 . An electrolyte composition 4 is disposed between the cathode 2 and the anode 3. The cathode 2 and anode 3 are housed in a reaction chamber 5, where the reaction chamber is closed system so that a positive pressure of nitrogen can be maintained within the chamber. In one aspect, the reaction chamber is composed of iron, stainless steel, or any other suitable metal that can withstand high pressure.

[0049] The cathode and anode can be composed of materials typically used in electrochemical reactions. In one aspect, the anode is composed of nickel, platinum, tungsten, a metal alloy, a metal oxide, or graphite. In another aspect, the cathode includes steel, nickel, copper, titanium, molybdenum, or graphite.

[0050] The electrochemical cell is operated under conditions such that metal ions in the electrolyte composition are converted to metal at the cathode 2, where the metal subsequently reacts with nitrogen gas to form metal nitride (MN). The metal nitride subsequently reacts with protons (H+) provided by the proton donor (HA) to form NH3, lithium ions and a deprotonated proton donor (A ). The protons (H+) are generated from the proton donor at the anode 3, the protons (H+) reacting with the deprotonated proton donor (A ) to produce the proton donor (HA).

[0051] The methods described herein provide an energy efficient way to produce ammonia. The only energy source required by the electrochemical cell is the voltage source. The methods described herein do not require providing heat to generate ammonia. For example, the temperature in the reaction chamber is from about 25 °C to about 30 °C.

[0052] In certain aspects, an ionically conductive membrane 6 is positioned between the anode 3 and the cathode 2. In one aspect, the membrane is composed of a porous glass or porous polymer such as porous polyethylene or porous polypropylene. In one aspect, the ionically conductive membrane is Daramic™ porous polymer separator (Polypore International Inc., Charlotte, NC, USA).

[0053] In one aspect, when an ionically conductive membrane is used, the electrolyte composition can first be added to the anode compartment (i.e., the space between the anode 3 and an ionically conductive membrane 6) then to the cathode compartment (i.e., the space between the cathode 2 and the ionically conductive membrane 6).

[0054] The electrochemical cell also includes a voltage source 7 connecting the cathode to the anode. In one aspect, the voltage source such as a DC power source or solar device for generating power is connected to the anode and cathode by a conductive wire 8 to supplyenergy to operate the electrochemical cell. In one aspect, the conductive wire is copper. In one aspect, the voltage source can apply a current having a current density from about -200 mA / cm2to about 2,500 mA / cm2. In another aspect, the voltage source can apply a current having a current density of -200 mA / cm2, 0 mA / cm2, 225 mA / cm2, 500 mA / cm2, 750 mA / cm2, 1 ,000 mA / cm2, 1 ,250 mA / cm2, 1 ,500 mA / cm2, 1 ,750 mA / cm2, 2,000 mA / cm2, 2,250 mA / cm2, or 2,500 mA / cm2, where any value can be a lower and upper endpoint of a range (e.g., 500 mA / cm2to 1 ,750 mA / cm2). The voltage source can be turned on and off as needed.

[0055] In certain aspects, the current can be applied to the electrochemical cell in an alternating fashion. In one aspect, (i) the current is applied for a first period of time, (ii), the current is turned off for second period of time, and (iii) the current is applied for a third period of time. In another aspect, the current can be changed throughout the reaction.

[0056] The electrolyte composition introduced into the electrochemical cell includes (i) an organic solvent, (ii) a proton donor, and (iii) a salt of lithium, calcium, magnesium, strontium, yttrium, scandium, zirconium, or any combination thereof positioned between the anode and cathode,

[0057] In one aspect, the salt of lithium, calcium, magnesium, strontium, yttrium, scandium, or zirconium is a tetrafluoroborate salt, a hexafluorophosphate a hexafluoroarsenate salt, a perchlorate salt, a triflate salt, a bisoxalato borate salt, a difluorooxalato borate salt, trifluorosulfonylimide salt, or a halide salt. The concentration of the salt can vary depending upon the solubility of the salt in the organic solvent. In one aspect, the salt of lithium, calcium, magnesium, strontium, yttrium, scandium, or zirconium has a concentration of about 0.5 M to about 4.0 M in the electrolyte composition. In another aspect, the salt of lithium, calcium, magnesium, strontium, yttrium, scandium, or zirconium has a concentration of about 0.5 M, 1 .0 M, 1.5 M, 2.0 M, 2.5 M, 3.0 M, 3.5 M, or 4.0 M, where any value can be a lower and upper endpoint of a range (e.g., 1 .0 M to 2.5 M).

[0058] The proton donor can be any compound or material that electrochemically generates protons. In one aspect, in the proton donor is an alcohol. In one aspect, the alcohol is a monohydroxyl compound or a polyhydroxyl compound (i.e, two or more hydroxyl groups). In another aspect, the proton donor is an alkyl alcohol, an aryl alcohol, or a cycloalkyl alcohol. In another aspect, the proton donor is a C1 to C7 alkyl alcohol. In another aspect, the proton donor is ethanol. In one aspect, the proton donor has a concentration of about 0.01 M to about 0.40 M in the electrolyte composition. In another aspect, the proton donor has a concentration of about 0.01 M, 0.05 M, 0.10 M, 0.15 M, 0.20 M, 0.25 M, 0.30 M, 0.35 M, or 0.40 M in the electrolyte composition, where any value can be a lower and upper endpoint of a range (e.g., 0.05 M to 0.25 M).

[0059] The organic solvent can be any solvent that will solubilize the salt of lithium, calcium, magnesium, strontium, yttrium, scandium, or zirconium. In one aspect, the solvent includes an ether such as, for example, dimethyl ether, diethyl ether, diglyme, or triglyme. In another aspect, the solvent includes a fluorinated solvent such as, for example, a perfluoro alkane or fluorobenzene. In another aspect, the organic solvent comprises tetrahydrofuran, dimethylsulfoxide, dimethoxyethane, N,N-dimethylformamide, or any combination thereof.

[0060] In one aspect, The electrolyte composition can be introduced into the reaction chamber of the electrochemical cell by a port. Referring to Figure 17, the port 9 with valve 10 can be used to introduce the electrolyte composition into the reaction chamber.

[0061] The amount of nitrogen gas introduced into the reaction chamber can vary on the selection of the salt and concentration of the proton donor. In one aspect, the pressure of the nitrogen gas in the electrochemical cell is from 1 bar to 100 bar. In another aspect, the pressure of the nitrogen gas in the electrochemical cell is 1 bar, 2 bar, 3 bar, 4 bar, 5 bar, 6 bar, 7 bar, 8 bar, 9 bar, 10 bar, 15 bar, 20 bar, 25 bar, 30 bar, 35 bar, 40 bar, 45 bar, 50 bar, 55 bar, 60 bar, 65 bar, 70 bar, 75 bar, 80 bar, 85 bar, 90 bar, 95 bar, or 100 bar, where any value can be a lower and upper endpoint of a range (e.g., 5 bar to 25 bar). In one aspect, nitrogen gas can be introduced into the reaction chamber of the electrochemical cell by a port. Referring to Figure 17, the port 1 1 with valve 12 can be used to introduce nitrogen gas into the reaction chamber. In another aspect, the reaction chamber can be fitted with pressure gauge for measuring the pressure within the reaction chamber.

[0062] In one aspect, the electrolyte composition includes a calcium salt and the proton donor comprises ethanol, and the pressure of nitrogen in the electrochemical cell is from 1 bar to 10 bar. In another aspect, the electrolyte composition includes calcium perchlorate and the proton donor comprises ethanol, and the pressure of nitrogen in the electrochemical cell is from 1 bar to 10 bar. In another aspect, the electrolyte composition includes calcium perchlorate, the proton donor comprises ethanol, the organic solvent comprises dimethoxymethane, and the pressure of nitrogen in the electrochemical cell is from 1 bar to 10 bar. In another aspect, the electrolyte composition includes a calcium salt, the proton donor comprises ethanol, the pressure of nitrogen in the electrochemical cell is from 1 bar to 10 bar, and the current applied to the electrochemical cell has a current density from about -5 mA / cm2to about -45 mA / cm2.

[0063] In one aspect, the electrolyte composition includes a magnesium salt and the proton donor comprises ethanol, and the pressure of nitrogen in the electrochemical cell is from 1 bar to 10 bar. In another aspect, the electrolyte composition includes magnesium perchlorate and the proton donor comprises ethanol, and the pressure of nitrogen in the electrochemical cellis from 1 bar to 10 bar. In another aspect, the electrolyte composition includes magnesium perchlorate, the proton donor comprises ethanol, the organic solvent comprises N,N- dimethylformamide, and the pressure of nitrogen in the electrochemical cell is from 1 bar to 10 bar. In another aspect, the electrolyte composition includes a magnesium salt, the proton donor comprises ethanol, the pressure of nitrogen in the electrochemical cell is from 1 bar to 10 bar, and the current applied to the electrochemical cell has a current density from about - 40 mA / cm2to about -20 mA / cm2.

[0064] In one aspect, the electrolyte composition includes a lithium salt and the proton donor comprises ethanol, and the pressure of nitrogen in the electrochemical cell is from 1 bar to 100 bar. In another aspect, the electrolyte composition includes lithium perchlorate, the proton donor comprises ethanol, and the pressure of nitrogen in the electrochemical cell is from 1 bar to 100 bar. In another aspect, the electrolyte composition includes lithium perchlorate, the proton donor comprises ethanol, the organic solvent comprises tetrahydrofuran, and the pressure of nitrogen in the electrochemical cell is from 1 bar to 100 bar. In another aspect, the electrolyte composition includes a lithium salt, the proton donor comprises ethanol, the pressure of nitrogen in the electrochemical cell is from 1 bar to 100 bar, and the current applied to the electrochemical cell has a current density from about -150 mA / cm2to about 600 mA / cm2.

[0065] The methods described herein can be performed by a batch or continuous process. In one aspect, the process can be performed by a batch process using a modified autoclave. Referring to Figure 9A, an autoclave with anode and cathode positioned within the autoclave can be used to produce ammonia.

[0066] In another aspect, the electrochemical cell described herein can be incorporated into system for continuously producing ammonia. Referring to Figure 18, a mixing tank 20 with the electrolyte composition is prepared. The tank 20 can have a port for introducing electrolyte into the tank. The tank 20 can be fitted with a motor to mix the electrolyte composition prior to introduction into the electrochemical cell 21 . In one aspect, the electrolyte composition can be introduced into electrochemical cell 21 under pressure with the use of a compressor or other pressurizing device. Separately, a nitrogen tank 22 is connected to the electrochemical cell 21 for introducing nitrogen into the electrochemical cell. In one aspect, nitrogen can be introduced into electrochemical cell 21 under pressure with the use of a compressor or other pressurizing device. A voltage source 27 is also connected to the electrochemical cell 21 .

[0067] After the electrolyte composition and nitrogen are introduced into the electrochemical cell, ammonia is produced. In one aspect, ammonia and electrolyte composition can be removed from the electrochemical cell 21 and directed into a device for removing or separating ammonia from the electrolyte composition. In one aspect, a gas liquid membrane 23 can beused to separate ammonia from the electrolyte composition. The electrolyte composition 24 after ammonia generation and separation has a reduced concentration of proton donor, where the conjugate base of the proton donor is present. The lean electrolyte composition 24 can be regenerated such that the conjugate base of the proton donor is converted to the protonated form so that it can be reused to produce additional ammonia. In one aspect, the lean electrolyte composition 24 can be fed into a proton donor regeneration system 25. In one aspect, the proton donor regeneration system 25 is an electrolysis stack, where oxygen and water are fed into the proton donor regeneration system 25 to convert the conjugate base of the proton donor to the proton donor. The regenerated electrolyte composition 26 can then be subsequently fed into the mixing tank 21 , where additional proton donor and / or salt can be added as needed.

[0068] Aspects

[0069] Aspect 1. A method for producing ammonia, the method comprising introducing nitrogen gas at a pressure greater than ambient pressure into an electrochemical cell, wherein the electrochemical cell comprises a reaction chamber; an anode and cathode positioned within the reaction chamber; a voltage source connecting the cathode to the anode; an ionically conductive membrane positioned between the cathode and the anode; and an electrolyte composition comprising (i) an organic solvent, (ii) a proton donor, and (Hi) a salt of lithium, calcium, magnesium, strontium, yttrium, scandium, zirconium, or any combination, wherein the electrolyte composition is positioned between the anode and cathode, wherein a current is applied to the electrochemical cell to convert nitrogen to ammonia.

[0070] Aspect 2. The method of Aspect 1 , wherein the pressure of the nitrogen gas in the electrochemical cell is from 1 bar to 100 bar.

[0071] Aspect 3. The method of Aspect 1 or 2, wherein the salt of lithium, calcium, magnesium, strontium, yttrium, scandium, or zirconium is a tetrafluoroborate salt, a hexafluorophosphate a hexafluoroarsenate salt, a perchlorate salt, a triflate salt, a bisoxalato borate salt, a difluorooxalato borate salt, trifluorosulfonylimide salt, or a halide salt.

[0072] Aspect 4. The method of any one of Aspects 1-3, wherein the salt of lithium, calcium, magnesium, strontium, yttrium, scandium, or zirconium has a concentration of about 0.5 M toabout 4.0 M in the electrolyte composition.

[0073] Aspect s. The method of any one of Aspects 1-4, wherein the proton donor comprises an alcohol.

[0074] Aspect s. The method of any one of Aspects 1-4, wherein the proton donor comprises an alkyl alcohol, an aryl alcohol, or a cycloalkyl alcohol.

[0075] Aspect ?. The method of any one of Aspects 1-4, wherein the proton donor comprises a C1 to C7 alkyl alcohol.

[0076] Aspect s. The method of any one of Aspects 1-4, wherein the proton donor comprises ethanol.

[0077] Aspect 9. The method of any one of Aspects 1-8, wherein the proton donor has a concentration of about 0.01 M to about 0.40 M in the electrolyte composition.

[0078] Aspect 10. The method of any one of Aspects 1-9, wherein the organic solvent comprises an ether.

[0079] Aspect 11. The method of Aspect 10, wherein the ether comprises dimethyl ether, diethyl ether, diglyme, or triglyme.

[0080] Aspect 12. The method of any one of Aspects 1-9, wherein the organic solvent comprises a fluorinated solvent.

[0081] Aspect 13. The method of Aspect 12, wherein the fluorinated solvent comprises a perfluoro alkane or fluorobenzene.

[0082] Aspect 14. The method of any one of Aspects 1-9, wherein the organic solvent comprises tetrahydrofuran, dimethylsulfoxide, dimethoxyethane, N,N-dimethylformamide, or any combination thereof.

[0083] Aspect 15. The method of any of Aspects 1-14, wherein the current applied to the electrochemical cell has a current density from about -200 mA / cm2to about 2,500 mA / cm2.

[0084] Aspect 16. The method of Aspect 1 , wherein the electrolyte composition comprises a calcium salt and the proton donor comprises ethanol, and the pressure of nitrogen in the electrochemical cell is from 1 bar to 10 bar.

[0085] Aspect 17. The method of Aspect 16, wherein the calcium salt is calcium perchlorate.

[0086] Aspect 18. The method of Aspect 16 or 17, wherein the organic solvent comprises dimethoxymethane.

[0087] Aspect 19. The method of any of Aspects 16-18, wherein the current applied to theelectrochemical cell has a current density from about -5 mA / cm2to about -45 mA / cm2.

[0088] Aspect 20. The method of Aspect 1 , wherein the electrolyte composition comprises a magnesium salt, the proton donor comprises ethanol, and the pressure of nitrogen in the electrochemical cell is from 1 bar to 10 bar.

[0089] Aspect 21. The method of Aspect 20, wherein the magnesium salt is magnesium perchlorate.

[0090] Aspect 22. The method of Aspect 20 or 21 , wherein organic solvent comprises N,N- dimethylformamide.

[0091] Aspect 23. The method of any of Aspects 20-22, wherein the current applied to the electrochemical cell has a current density from about -40 mA / cm2to about -20 mA / cm2.

[0092] Aspect 24. The method of Aspect 1 , wherein the electrolyte composition comprises a lithium salt, the proton donor comprises ethanol, and the pressure of nitrogen in the electrochemical cell is from 1 bar to 100 bar.

[0093] Aspect 25. The method of Aspect 24, wherein the lithium salt is lithium perchlorate.

[0094] Aspect 26. The method of Aspect 24 or 25, wherein organic solvent comprises tetrahydrofuran.

[0095] Aspect 27. The method of any of Aspects 24-26, wherein the current applied to the electrochemical cell has a current density from about -150 mA / cm2to about 600 mA / cm2.

[0096] Aspect 28. The method of any one of Aspects 16-27, wherein ethanol has a concentration of about 0.03 M to about 0.08 M in the electrolyte composition.

[0097] Aspect 29. The method of any of Aspects 1-28, wherein the anode comprises nickel, platinum, tungsten, a metal alloy, a metal oxide, or graphite.

[0098] Aspect 30. The method of any of Aspects 1-29, wherein the cathode comprises steel, nickel, copper, titanium, molybdenum, or graphite.

[0099] Aspect 31. The method of any of Aspects 1-30, wherein the ionically conductive membrane comprises a glass or polymer.

[0100] Aspect 32. The method of Aspect 31 , wherein the polymer comprises polyethylene or polypropylene.

[0101] Aspect 33. The method of any of Aspects 1-32, wherein the temperature in the reaction chamber is from about 25 °C to about 30 °C.

[0102] Aspect 34. The method of any of Aspects 1 -33, wherein (i) the current is applied for afirst period of time, (ii), the current is turned off for second period of time, and (iii) the current is applied for a third period of time.

[0103] Aspect 35. The method of any of Aspects 1-34, wherein the electrolyte composition is stirred in the electrochemical cell.

[0104] Aspect 36. The method of any of Aspects 1-35, wherein the reaction chamber comprises stainless steel.

[0105] Aspect 37. The method of any of Aspects 1-36, wherein the reaction chamber comprises a first port for introducing the electrolyte composition into the reaction chamber and a second port for introducing nitrogen into the reaction chamber.

[0106] Aspect 38. The method of any of Aspects 1-37, wherein the reaction chamber comprises a pressure gauge for measuring the pressure within the reaction chamber.

[0107] Aspect 39. The method of any of Aspects 1-38, wherein after ammonia is produced, removing the ammonia from the reaction chamber.

[0108] Aspect 40. The method of any of Aspects 1-39, wherein after ammonia is produced, (i) removing electrolyte composition comprising ammonia from the reaction chamber and (ii) removing ammonia from the electrolyte composition.

[0109] Aspect 41. The method of Aspect 40, wherein after removal of ammonia, the electrolyte composition is regenerated and introduced into the reaction chamber.

[0110] Aspect 42. The method of any of Aspects 1-41 , wherein the method is performed continuously to produce ammonia.EXAMPLES

[0111] The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how the compounds, compositions, articles, devices and / or methods claimed herein are made and evaluated, and are intended to be purely exemplary of the disclosure and are not intended to limit the scope of what the inventors regard as their disclosure. Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperature, etc.), but some errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, temperature is in °C or is at ambient temperature, and pressure.

[0112] Example 1 - Lithium System

[0113] Materials and Methods

[0114] Electrochemical Experiments

[0115] All the electrochemical ammonia synthesis experiments were conducted in 2-electrode single compartment glass cell enclosed in an electrochemical custom-designed autoclave placed in an explosion-proof box. For the benchmarking experiments and effect of pressure, the electrolyte solutions consisted of 2M LiCIC dissolved in THF containing 0.065 M EtOH is the electrolyte. For studying the effect of concentration of EtOH, following solution concentrations were used, 2M I CIO4 + 0.0325M EtOH, 2M I CIO4 + 0.0487 M EtOH, 2M LiCIO4+ 0.065M EtOH, 2M LiCIO4+ 0.097 M EtOH, 2 M LiCIO4+ 0.13 M EtOH, 2 M LiCIO4+ 0.1625 M EtOH, 2 M LiCIO4+ 0.325 M EtOH in THF. Solution concentrations to investigate the effect of different proton sources: 2 M LiCIO4+ 0.1 M MeOH, 2 M LiCIO4+ 0.1 M EtOH, 2 M UCIO4 + 0.1 M 1-PrOH, 2 M LiCIO4+ 0.1 M 1-BuOH, 2 M LiCIO4+ 0.1 M 1-PeOH in THF. For the experiments involving LiBF4, 0.065 M EtOH in THF was used with varying concentration of LiBF4(1 M, 2 M, 3 M, 4 M) was used. The electrolyte solution was presaturated with N2using a mass flow controller at a flow rate of 200 seem for 10 minutes. Ni foam is used as the cathode. Fresh Ni foam was used for every run and prior to electrochemical tests, it was rinsed with EtOH for 10 minutes to eliminate water contamination followed by oven drying at 80 °C for 20 minutes. A planar Pt anode was used. After every reaction, it was sonicated in acetone for 15 mins, followed by sonication in ethanol for another 15 mins and then oven drying at 80 °C for 20 minutes. The dried Ni foam and Pt anode were spot-welded with Cu wire and Cu tape for good electrical connection. The single compartment glass cell and a magnetic stirring bar is dried at 80 °C for 1 hour. The anode and cathode are ~1 cm apart and the surface area of the cathode facing the anode is 4 cm2. After assembling the cell in the autoclave, it was flushed with N2directly from the gas cylinder 10 times to remove all the oxygen and other atmospheric contaminants present inside the autoclave prior to an electrochemical experiment. It was then transferred in the glove box along with the air compressor. Finally, the pressure is increased up to 20 bars and de-pressurized thrice in order to flush out any remaining atmospheric contaminants, then filled to desired pressure. Subsequently, electrochemical experiments were carried out by using a potentiostat (Biologic SP 300) and the set up was stirred at 700 rpm. Chronopotentiometry was done with switching current strategy with -150 mA / cm2for 2 min, followed by 0 mA / cm2(hereafter denoted resting potential) for 2 min, depending on whether the WE potential needed to be increased, decreased, or stabilized. We note that all experiments were conducted at room temperature.

[0116] Colorimetric quantification of products

[0117] NH3was quantified by the Indophenol blue method22using a standard additions method11to account for the changes in appearance and properties of the electrolyte solution after the electrochemical reaction. In this method, post electrolyte sample is diluted 100-400folds with 0.1 M H2SO4 to bring the concentration to the detectable limits. After dilution the ammonia present will be in the form of ammonium ion. Subsequently, the solution is centrifuged twice for 15 minutes each to obtain a clear solution. Calibration solutions are prepared using 0.01 M NH4CI as the stock solution and further diluting it to make 100-500 M NH4CI calibration solutions in H2SO4. The blank solution for prepared by adding, 500 L of H2SO4, 500 pL of Phenol nitroprusside and 500 pL of alkaline sodium hypochlorite. 6 sample vials were each filled with 400 pL aliquots of the centrifugated diluted post electrolyte solution and 100 pL of internal aqueous standard solutions (0.1 M H2SO4, 100 pM-500 pM NH4CI in 0.1 M H2SO4) along with 500 pL of Phenol nitroprusside followed by 500uL of alkaline sodium hypochlorite. The mixtures were incubated in dark for 30 minutes at ambient temperature. The sample changes color from colorless to blue. The sample was scanned for absorbance as a function of wavelengths from 400 to 800 nm using a visible spectrometer (Genesys 30 Visible Spectrometer). The maximum absorbance was observed at 632 nm and hence 632 nm was chosen to measure absorbances to quantify NH3. The ratio of the intercept and the slope is the moles of ammonia in the test sample. After calculating the moles, concentration and partial current density is determined.

[0118] Control experiments

[0119] To perform an Ar blank experiment, the electrolyte was pre-saturated with Ar instead of N2, and after injection into the autoclave cell, the pumping and purging procedure was carried out with Ar instead of N2. An electrochemical cycling experiment with switching was carried out which had almost no ammonia current. Experiments with switching currents and standard electrolyte composition except the 0.1 M EtOH were also carried out and no ammonia current was observed. To contemplate the effect of switching currents, an electrochemical run was done with straight 2 hours of working time and no switching.

[0120] Energy Efficiency Calculations

[0121] The energy efficiency for Li-mediated ammonia synthesis is defined as the ratio between the amount of energy contained in ammonia produced during the reaction and total energy used by the Li-mediated system via potentiostat.AG x i x t EE = - n x F x V x Q where F is Faraday’s constant, V is the total cell voltage, i is the NH3current, AG is the Gibbs free energy of the NH3oxidation reaction (339 kJ / mol), Q is the total charge passed and n is the number of electrons transferred during the reaction for each mole of NH3. The voltage efficiency is calculated by using the total current instead of the NH3current.

[0122] Results and Discussions

[0123] Benchmarking LiMAS - Optimal Switching and Potential-Independent Selectivities

[0124] Benchmarking experiments were performed at 6 bar and room temperature using a two-electrode cell without a separator placed in a modified autoclave setup. Details of the high-pressure electrochemical cell are provided in the supporting information. Here, Ni foam was used as the cathode, and Pt was used as the anode. The electrolyte is composed of 2M LiCICu and 0.1 M EtOH dissolved in THF. The current switching strategy with 2 min working time and 2 min resting time was utilized to stabilize SEI. The total run time was 4 h. The resting current density was kept at 0 mA / cm2(i.e., open circuit), and different working current densities were applied. The NH3was quantified by using the method of additions.11

[0125] When the applied current density (CD) is increased, the NH3CDs increase while the NH3FES remain constant at an average value of ~15 % (Error! Reference source not found.A and 1 B). This indicates that the NH3and H2production rate increases proportionally with increasing current density. This also implies that the ratio of Li protolysis to Li3N protolysis rates is independent of potential. Therefore, the potential-independent behavior could be due to a fixed fractional coverage of Li3N on the cathode since protolysis is a thermochemical step. The coverage of Li3N is balanced by nitridation and protolysis reactions, which in turn are directly related to N2pressure and proton donor, respectively. In addition to these two factors, the anion of Li salt, its concentration, and the solvent also affect the kinetics of LiMAS. Hence, we investigate those parameters.

[0126] Switching time plays a role in stabilizing the cell voltage by maintaining the stability of the SEI. Li deposition occurs primarily during working time (when the negative current density is applied). Li nitridation, Li3N protolysis, and Li protolysis are the rate-limiting steps. During the resting time (when the current density is zero), Li deposition halts, and the nitridation and protolysis steps dominate. Li nitridation is dependent on the N2pressure. Li3N protolysis and Li protolysis are dependent on the proton donor concentration. NH3FE is dependent on both Li nitridation and Li3N protolysis. Hence, we believe that the N2pressure and EtOH concentration parameters are interdependent. It takes a while to develop the stable solid electrolyte interface (around 1 h) that forms a passivating layer around the cathode, thereby preventing electrolyte oxidation. The passivating layer is permeable only to Li+ions, N2, H2, NH3, and H+, preventing the THF from contacting the electrode.

[0127] Switching times also affect mass transport. It is experimentally found that at resting time, the open circuit potential should be close to ~3V (equilibrium reduction potential of theLi+) for a stable SEI and NH3formation. If the open circuit potential goes below 3V, then H2formation is favored by Li protolysis. Hence, the resting time should be optimal to maintain the open circuit potential close to ~3V. Stirring speed also affects Li3N and Li protolysis, and it could vary depending on the size and type of stirrer used. If the open circuit potential goes very less than 3 V, it can be brought back to 3 V by reducing the resting time, the working time, and the stirring speed. If the open circuit potential goes higherthan 3 V, it can be brought back to 3 V by increasing the resting time, decreasing the working time, and increasing the stirring speed. For stabilizing the SEI, 2 min of working time and 2 min of resting time is optimal as per our observation. At higher applied current densities, there are many oscillations in the total cell potential due to the generated heat, resulting in substantial error bars. For further studies, we used -150 mA / cm2with 2 min working and 2 min resting times as the system oscillations are less, and it is easy to perform those experiments on a lab scale.

[0128] Effect ofN2Pressure and Proton Donor Concentration

[0129] To evaluate the effect of N2pressure, the NH3FE and NH3CD were measured for increasing N2pressures from 1 bar to 100 bar at a fixed EtOH concentration of 0.065 M. Error! Reference source not found.A and 2B show the NH3FE and CD as a function of pressure. As the pressure varied from 1 bar to 15 bar, the NH3FE increased gradually with a steeper rise from 15 bar to 20 bar, beyond which the NH3FE remained constant despite raising the N2pressure up to 100 bar. As the pressure increases, the N2solubility in the electrolyte solution increases, increasing the concentration of dissolved N2which reacts with the electrodeposited Li to form Li3N. The Li3N is then protonated by ethanol to form NH3and Li+ion. The increasing pressure ensures excess availability of Li3N. Therefore, the rate of protonation is limited by the EtOH mass transport. Hence, despite raising the pressures to 100 bar to increase the concentration of dissolved N2and Li3N, there is no change in NH3FE. The maximum NH3FE obtained was ~35 % at an optimal pressure of 20 bar.

[0130] At pressures greater than 20 bar, the reaction is in the EtOH limited (i.e., H+limited) regime. It has been previously proposed that the NH3FE increases with increasing pressures and proton donor concentrations. However, as shown in Fig. 2, there is a limiting trend. We start by studying the effect of ethanol concentration at 20 bar N2pressure. As the ethanol concentration increases, the NH3FE and CD increase up to 0.065 M ethanol concentration, beyond which the NH3FE and CD decrease. The decrease in FE and CD can be associated with increased Li protonation instead of Li3N protonation. The optimal ethanol concentration they observed was 0.065 M for maximum NH3FE. At higher pressures such as 50 bar and 100 bar, the trend remains the same and the maximum NH3FE remains at ~35 % at an optimalethanol concentration of 0.065 M. Error! Reference source not found, shows the effect of proton concentrations on the NH3FE and NH3CD.

[0131] Effect of Proton Donor Type and pKa

[0132] The availability of protons depends on the concentration of the proton donor and the pKa of alcohol. To investigate the effect of pKa, different linear chain alcohol proton sources such as methanol (pKa = 15.5), ethanol (pKa = 15.9), 1-propanol (pKa = 16.85), 1-butanol (pKa = 17), and 1-pentanol (pKa = 16.84) are used in LiMAS. The concentration of the proton sources was kept at 0.1 M and N2pressure was maintained at 20 bar. Figure 4 shows the NH3FE and CD as a function of different proton sources. 1 -Butanol shows the highest NH3FE (20.762 %) followed by ethanol (15.46 %), 1-propanol (13.94 %), 1-pentanol (1 1.80 %) and methanol (8.98 %). The variation in FEs seems to follow the pKa trend. The pKa also increases with increasing alkyl chain length from methanol to 1-butanol and then decreases. The role of the proton donors and their influence on NH3FE is not well understood in the literature. The proton donor could influence the SEI by allowing optimal diffusion of the N2and the proton donor to reach the surface of the electrode, thereby promoting NH3FE. In-situ analysis of the SEI is required to understand the role of proton donors in determining the NH3selectivity.

[0133] Effect of Li Salt Anion

[0134] The optimal conditions for maximum NH3selectivity are 20 bar N2pressure and 0.065 M ethanol. So far, the experiments have been performed with LiCIO4 / THF electrolytes. The size of an anion in 1 :1 electrolyte plays a significant role in altering the solvation structure of Li+ions and the potential distribution in the electric double layer. According to the sizedependent Poisson Boltzmann equation, increasing the size of an anion will decrease the local density of Li+as well as the local electric field near the cathode. It is well from Li metal battery literature that SEI becomes stable (i.e., non-dendritic Li metal growth) when the gradient of Li+and the electric field near the cathode are not high.19To probe the anion effect, we change the anion from CIO4- of size ~0.24 nm to BF4- of size ~0.3 nm. The optimal conditions for LiMAS are tested for LiBF4 / THF electrolyte. LiBF4is expected to give a higher performance by stabilizing the SEI. However, LiBF4is expensive and very sensitive to moisture. Error! Reference source not found, shows the NH3FE and NH3CD as a function of the LiBF4concentration. As the concentration of LiBF4is increased, the NH3FE and NH3CD increase due to stable Li deposition and increased conductivity of the electrolyte. A maximum NH3FE of ~70% and NH3CD of ~100 mA / cm2is observed at 3 M LiBF4concentration. Beyond 3 M LiBF4concentration, the NH3FE and NH3CD drop down as the viscosity of the solution increases at higher concentrations, thereby reducing the conductivity of the solution.

[0135] Energy Efficiencies ofLiMAS

[0136] The energy efficiencies are calculated by multiplying faradaic efficiency and voltage efficiency for all the process conditions studied above. The voltage efficiency is the ratio of the equilibrium potential of the redox reaction and the actual cell voltage. Typically, the equilibrium potential is chosen for the overall redox reaction occurring in the cell. Depending on the source of protons, it could be one of the following redox reactions.N2+ 3 H22 NH3E° = 0.057 V vs SHE orN2+ 3 H2O — > 2 NH3+ 3 / 2 O2E° = 1.172 V vs SHE

[0137] However, neither used H2nor H2O was used in this study to generate protons. The protons were most likely generated from EtOH or THF oxidation at the anode. Since the equilibrium potential for EtOH oxidation to CO2is ~0.10 V vs. SHE, and THF to THF-OH is 0.85 V vs. SHE, the equilibrium potential for NH3synthesis is 0.043 V vs. SHE for EtOH and is ~0.8 V vs. SHE for THF as proton sources. In an ideal case, the protons should be derived from water splitting, yielding the highest voltage efficiency among all the proton sources. Error! Reference source not found, shows the experimentally measured faradaic efficiency, voltage efficiency obtained using 1.172 V vs. SHE (i.e., water as proton source), and energy efficiency for different operating conditions. The energy efficiency does not change much as most of the experiments were conducted at fixed working current density, for which the total cell potential remains almost the same for all the studied conditions. The maximum observed energy efficiency is ~9 %. The lower energy efficiency is primarily due to lower voltage efficiency. This is because the electrochemical cell used in this study was a vial with electrodes dipped in the electrolyte, which has excessive ohmic losses. The details of the energy efficiency calculations are provided in the methods section.

[0138] The voltage efficiency mostly determines the energy efficiency of LiMAS. The voltage efficiency strongly depends on the proton source, such as H2O, H2, alcohols, or other hydrogen carrier. Figure 7 shows the voltage efficiency versus cell potential for various proton sources. It can be observed that the highest efficiency can be obtained when protons are derived from H2O. However, it is challenging to utilize H2O in LiMAS. The voltage efficiency decreases in the following order of the proton sources - H2O » H2> Alcohols. Also, to prevent Li metal oxidation, the operating potentials must be lower than the equilibrium potential of Li Li in THF, which is _2.98 V vs. SHE.20The minimum cell potential with H2O oxidation at anode and Li deposition at the cathode is, therefore 1.23 V - (-2.98 V) = 4.21 V. This limits the energy efficiency of LiMAS at 27.83%.

[0139] Techno-Economic Analysis

[0140] A previously reported21preliminary techno-economic model of LiMAS was utilized for green NH3production scaled up linearly to an industrial level of 1 ,000,000 tons per annum, assuming no benefits of economics of scale. The reported CapEx estimates of $384 per ton21at 4 mA / cm2were scaled linearly with NH3current density. Since the operating pressures in this study are higher (~ 20 bar) than the system used in previous techno-economic analysis, we also considered additional CapEX costs associated with high-pressure autoclave-based electrochemical cells for continuous operation. We assumed a single autoclave unit as 40 stacks of 500 electrochemical cells of 0.2 m2area electrode connected with a single high- pressure line. The additional CapEX cost associated with one high-pressure autoclave is $100,000. The number of autoclave units required scales linearly with current density. Fig.8 A shows a drastic reduction in CapEx cost with increasing current density. At an optimal NH3current density of 100 mA / cm2, the estimated CapEx is $84 per ton of NH3, which is 78% less than the ambient LiMAS process($384 per ton)21and 84% less than the modified green Haber- Bosch process ($522 per ton).

[0141] The OpEx cost includes the cost of electricity to operate autoclaves and the cost of utilities to operate ancillary equipment. The cost of electricity was assumed to be $10 / MW-hr, while the utility requirement for ancillary equipment was fixed at $88 per ton21of NH3. The OpEx cost increases linearly with the cell voltage, as shown in Fig. 8B. As shown in Fig. 7, LiMAS requires a minimum of 4.21 V with H2O as a proton source and 2.98 V with H2 as a proton source. This results in the bare minimum OpEx cost of $287 perton with H2O and $229 per ton with H2 as proton sources. Considering the required overpotential losses at the anode and cathode, a cell voltage of 6 V is a reasonable operating potential for practical purposes. At the operating potential of 6V, the OpEx cost is $372 perton with energy efficiency of ~20%. This projected OpEx cost is 53% less than the ambient pressure LiMAS ($790 per ton) and 18% less than the modified green Haber-Bosch process ($454 per ton).

[0142] The operation of the LiMAS plant at 100 mA / cm2of NH3current density and cell voltage of 6V would result in a green NH3cost of $456 per ton, which would enable a >61 % reduction in NH3cost from ambient LiMAS and >53% reduction in NH3from modified Green Haber- Bosch system of the same scale. This substantial reduction in both capital and operational expenditures underscores the economic viability of high-pressure LiMAS on a large scale. The findings point to the potential for a profound shift in the ammonia production industry, which promises environmental benefits and significant economic advantages. We anticipate the market price of green NH3production based on this process to be feasible below <$400 / ton,including a sub-four-year discounted payback period. The target price with this technology going forward is below $100 / ton.

[0143] Conclusions

[0144] Li-mediated ammonia synthesis (LiMAS) is a promising approach to synthesizing NH3electrochemically. The NH3synthesis scheme in LiMAS involves three important steps: electrodeposition of Li+, Li nitridation, and Li3N protolysis. The electrodeposition of Li+followed by stabilization of the solid-electrolyte interface (SEI) is the most important part of LiMAS to ensure stable operation and cell voltage. The LiMAS reactor is operated with a square wave current profile where working current density is applied for 2 minutes, followed by a 2-minute resting period at open circuit voltage. The square wave current profile helps in improving SEI and voltage stability. It also balances the reaction time for Li nitridation during the working period and Li3N protolysis during the resting period, which also governs the coverage of Li3N.

[0145] A side reaction involving Li protolysis also results in H2formation that suppresses Li nitridation. Since both rate-limiting steps - Li protolysis for H2and Li3N protolysis for NH3are thermochemical, the NH3FE does not change with increasing cell voltage or applied current density. However, the availability of N2and protons can directly affect Li nitridation and Li3N protolysis, thereby affecting NH3selectivity. As the pressure is increased up to 20 bar, the NH3FE increases and reaches a maximum value of ~35 %. Beyond 20 bar, the N2pressure has no effect on NH3FE, which indicates that the Li3N protolysis could be limited by the mass transport of proton donors. Increasing the concentration of proton donors reduces the mass transfer limitation and shows improvement in NH3FE. However, after a certain concentration limit, the further increase in proton donor concentration decreases the NH3FE, which could be due to the increased rate of Li protolysis. Similar behavior is observed at all three pressures studied (20, 50, and 100 bar), and higher ethanol concentration did not improve the performance at higher pressures. The pKa of the proton donor can also affect the availability of protons. The pKa increases with increasing the alkyl chain length of the proton donor from methanol to 1-butanol and then decreases for 1-pentanol. The NH3FE also follows the same trend, yielding maximum FE for 1-butanol. The decrease in the diffusion coefficient with increasing alkyl chain length can also have some effect on the mass transfer of proton donor and NH3FE.

[0146] The anion of Li salt can also affect the stability of SEI and NH3FE. Increasing the anion size decreases the Li+gradient and electric field at the cathode, which has been previously reported to favor the dendrite-free deposition of Li metal. Replacing the CK r anion with a larger BF4- anion improved the performance of LiMAS. A maximum NH3FE of ~70 % and a maximum NH3current density of ~- 100 mA / cm2is observed at an applied current densityof -150 mA / cm2, when 0.065 ethanol concentration, 20 bar N2pressure, and 3 M LiBF4are used. The role of SEI in understanding the NH3FE is currently not understood well in the literature and in-situ studies are required, such as ambient pressure XPS and neutron reflectivity studies to probe the SEI in-situ and perhaps under operando conditions.

[0147] The source of proton in LiMAS plays a crucial role in achieving higher energy efficiency. The requirement of keeping the cathodic potential under the equilibrium potential of Li plating (i.e., -2.98 V vs SHE) limits the achievable energy efficiency. The energy efficiency decreases in the H2O >> H2> alcohols. The maximum energy efficiency achievable is 27.83% using H2O as a proton source, but it is exceedingly difficult to realize such LiMAS system. A preliminary techno-economic analysis shows that high-pressure LiMAS is significantly better than ambient LiMAS and modified Haber-Bosch process. The operation of the LiMAS plant at 100 mA / cm2of NH3current density and cell voltage of 6V will yield green NH3at an all-inclusive cost of $456 per ton, which enables >61% reduction in NH3cost from ambient LiMAS and >53% reduction in NH3from modified Green Haber-Bosch system of the same scale. This substantial reduction in both capital and operational expenditures underscores the economic viability of high-pressure LiMAS on a large scale.

[0148] References for Example 1(1) Erisman, J. W.; Sutton, M. A.; Galloway, J.; Klimont, Z.; Winiwarter, W. How a century of ammonia synthesis changed the world. Nature geoscience 2008, 1 (10), 636-639.(2) Wang, L.; Xia, M.; Wang, H.; Huang, K.; Qian, C.; Maravelias, C. T.; Ozin, G. A. Greening ammonia toward the solar ammonia refinery. Joule 2018, 2 (6), 1055-1074.(3) Andersen, S. Z.; Statt, M. J.; Bukas, V. J.; Shapel, S. G.; Pedersen, J. B.; Krempl, K.; Saccoccio, M.; Chakraborty, D.; Kibsgaard, J.; Vesborg, P. C. Increasing stability, efficiency, and fundamental understanding of lithium-mediated electrochemical nitrogen reduction. Energy & Environmental Science 2020, 13 (1 1), 4291-4300.(4) Lazouski, N.; Schiffer, Z. J.; Williams, K.; Manthiram, K. Understanding continuous lithium- mediated electrochemical nitrogen reduction. Joule 2019, 3 (4), 1 127-1 139.(5) Fichter, F.; Girard, P.; Erlenmeyer, H. Elektrolytische Bindung von komprimiertem Stickstoff bei gewbhnlicher Temperatur. Helvetica Chimica Acta 1930, 13 (6), 1228-1236.(6) Tsuneto, A.; Kudo, A.; Sakata, T. Lithium-mediated electrochemical reduction of high pressure N2 to NH3. Journal of Electroanalytical Chemistry 1994, 367 (1-2), 183-188.(7) Lazouski, N.; Chung, M.; Williams, K.; Gala, M. L; Manthiram, K. Non-aqueous gas diffusion electrodes for rapid ammonia synthesis from nitrogen and water-splitting-derived hydrogen. Nature Catalysis 2020, 3 (5), 463-469.(8) Lazouski, N.; Steinberg, K. J.; Gala, M. L.; Krishnamurthy, D.; Viswanathan, V.; Manthiram, K. Proton donors induce a differential transport effect for selectivity toward ammonia in lithium- mediated nitrogen reduction. / ACS Catalysis 2022, 12 (9), 5197-5208.(9) Cherepanov, P. V.; Krebsz, M.; Hodgetts, R. Y.; Simonov, A. N.; MacFarlane, D. R. Understanding the factors determining the faradaic efficiency and rate of the lithium redox- mediated N2 reduction to ammonia. The Journal of Physical Chemistry C 2021 , 125 (21), 11402-1 1410.(10) Rakov, D.; Hasanpoor, M.; Baskin, A.; Lawson, J. W.; Chen, F.; Cherepanov, P. V.; Simonov, A. N.; Howlett, P. C.; Forsyth, M. Stable and Efficient Lithium Metal Anode Cycling through Understanding the Effects of Electrolyte Composition and Electrode Preconditioning. Chemistry of Materials 2021 , 34 (1), 165-177.(1 1) Suryanto, B. H.; Matuszek, K.; Choi, J.; Hodgetts, R. Y.; Du, H.-L.; Bakker, J. M.; Kang, C. S.; Cherepanov, P. V.; Simonov, A. N.; MacFarlane, D. R. Nitrogen reduction to ammonia at high efficiency and rates based on a phosphonium proton shuttle. Science 2021 , 372 (6547), 1187-1191.(12) Du, H.-L.; Chatti, M.; Hodgetts, R. Y.; Cherepanov, P. V.; Nguyen, C. K.; Matuszek, K.; MacFarlane, D. R.; Simonov, A. N. Electroreduction of nitrogen with almost 100% current-to- ammonia efficiency. Nature 2022, 609 (7928), 722-727.(13) Sazinas, R.; Andersen, S. Z.; Li, K.; Saccoccio, M.; Krempl, K.; Pedersen, J. B.; Kibsgaard, J.; Vesborg, P. C. K.; Chakraborty, D.; Chorkendorff, I. Towards understanding of electrolyte degradation in lithium-mediated non-aqueous electrochemical ammonia synthesis with gas chromatography-mass spectrometry. RSC advances 2021 , 11 (50), 31487-31498.(14) Sazinas, R.; Li, K.; Andersen, S. Z.; Saccoccio, M.; Li, S.; Pedersen, J. B.; Kibsgaard, J.; Vesborg, P. C.; Chakraborty, D.; Chorkendorff, I. Oxygen-Enhanced Chemical Stability of Lithium-Mediated Electrochemical Ammonia Synthesis. The Journal of Physical Chemistry Letters 2022, 13 (20), 4605-461 1 .(15) Li, K.; Andersen, S. Z.; Statt, M. J.; Saccoccio, M.; Bukas, V. J.; Krempl, K.; Sazinas, R.; Pedersen, J. B.; Shadravan, V.; Zhou, Y. Enhancement of lithium-mediated ammonia synthesis by addition of oxygen. Science 2021 , 374 (6575), 1593-1597.(16) Li, K.; Shapel, S. G.; Hochfilzer, D.; Pedersen, J. B.; Krempl, K.; Andersen, S. Z.; Sazinas, R.; Saccoccio, M.; Li, S.; Chakraborty, D. Increasing Current Density of Li-Mediated Ammonia Synthesis with High Surface Area Copper Electrodes. ACS Energy Letters 2021 , 7 (1), 36-41.(17) Li, S.; Zhou, Y.; Li, K.; Saccoccio, M.; Sazinas, R.; Andersen, S. Z.; Pedersen, J. B.; Fu, X.; Shadravan, V.; Chakraborty, D. Electrosynthesis of ammonia with high selectivity and high rates via engineering of the solid-electrolyte interphase. Joule 2022, 6 (9), 2083-2101.(18) Fu, X.; Pedersen, J. B.; Zhou, Y.; Saccoccio, M.; Li, S.; Sazinas, R.; Li, K.; Andersen, S. Z.; Xu, A.; Deissler, N. H. Continuous-flow electrosynthesis of ammonia by nitrogen reduction and hydrogen oxidation. Science 2023, 379 (6633), 707-712.(19) Xu, X.; Liu, Y.; Hwang, J. Y.; Kapitanova, O. O.; Song, Z.; Sun, Y. K.; Matic, A.; Xiong, S. Role of Li-Ion depletion on electrode surface: underlying mechanism for electrodeposition behavior of lithium metal anode. Advanced Energy Materials 2020, 10 (44), 2002390.(20) Westhead, O.; Tort, R.; Spry, M.; Rietbrock, J.; Jervis, R.; Grimaud, A.; Bagger, A.; Stephens, I. L. The origin of overpotential in lithium-mediated nitrogen reduction. Faraday Discussions 2023.(21) Gomez, J. R.; Garzon, F. Preliminary economics for green ammonia synthesis via lithium mediated pathway. International Journal of Energy Research 2021 , 45 (9), 13461-13470.(22) Andersen, S. Z.; Colic, V.; Yang, S.; Schwalbe, J. A.; Nielander, A. C.; McEnaney, J. M.; Enemark-Rasmussen, K.; Baker, J. G.; Singh, A. R.; Rohr, B. A. A rigorous electrochemical ammonia synthesis protocol with quantitative isotope measurements. Nature 2019, 570 (7762), 504-508.

[0149] Example 2 - Calcium System

[0150] Materials and Methods

[0151] Electrochemical Experiments

[0152] The electrochemical synthesis of ammonia was conducted using a specialized 2- electrode setup, comprising a single-compartment glass cell within a custom-designed autoclave placed in an explosion-proof enclosure. The salt used in the calcium mediated reaction reaction was dried in a vacuum oven for 24 hours to remove any moisture. The sealed Ca salt was opened in the glovebox, and was used to prepare the electrolyte under an Argon atmosphere. An oxygen sensor (ATO-GD200-O2) was placed inside the glove box to monitor the oxygen levels. The electrolyte utilized was a solution of 0.5 M dried calcium perchlorate tetrahydrate dissolved in dimethoxyethane (DME), with an addition of 0.065 M ethanol (EtOH), pre-saturated with nitrogen (N2) for 10 minutes at a flow rate of 200 standard cubic centimetersper minute (seem). Nickel (Ni) foam was employed as the cathode, with fresh Ni foam used for each run. Before the reaction, the Ni foam was cleansed with ethanol to eliminate water contamination, followed by drying in an oven at 80 °C for 20 minutes. A planar platinum (Pt) anode was utilized, cleaned after each reaction by sonication in acetone and ethanol, followed by oven drying at 80°C for 20 minutes. The Ni foam and Pt anode were electrically connected using copper (Cu) wire and Cu tape. The glass cell and magnetic stirring bar were dried at 80°C for 1 hour before assembly. The distance between the anode and cathode was approximately 1 cm, with a cathode surface area of 4 cm2facing the anode. Before electrochemical experiments, the autoclave was purged with N2from a gas cylinder 10 times to remove oxygen and other contaminants, and then pressurized to a desired pressure of 6 bars. All the gases are passed through purifiers (Vici metronics - P300-1) at the mainline before supply. Electrochemical experiments were carried out using a potentiostat (Biologic SP 300) with stirring at 700 rpm. Chronopotentiometry was conducted with varying current density ranging from -5 mA / cm2up to -50 mA / cm2for 2 hours. All experiments were conducted at room temperature.

[0153] NMR

[0154] For the NMR analyses conducted on the Bruker Neo 600 MHz system with QCI-F cryoprobe, dimethylsulfoxide-d6 (DMSO-d6) was utilized as the deuterated solvent. For the preparation of the sample, 1 mL of post-electrolyte was mixed with 1 mL of 0.1 M H2SO4 to convert all the NH3 into ammonium sulphate. The mixture was thoroughly mixed and the solvent was then evaporated using a rotary evaporator(Across international 2L). Once the remaining solvent was reduced to 1 / 5 of its original volume, water was added to bring it back up to the original volume. The resulting solution was thoroughly mixed and centrifuged. Subsequently, 570 L of the solution was added to the NMR tube, along with 30 L of DMSO- d6, for further analysis. Calibration solutions using NH4CI and15NH4CI were prepared in the same way from 2mM to 10mM. Subsequently, the resulting solution was transferred into an NMR tube for testing. The measurements were performed using a Bruker spectrometer equipped with a cryoprobe. The data presented herein represent the accumulation of 16 scans. To suppress the water resonance, the excitation sculpting method employing a 3-ms 180° shaped pulse centered at 4.612 ppm was employed. The perfect-echo variant was selected to minimize J-modulation for samples analyzed at 500 MHz. A total of 1 ,024 transient scans were recorded with an interscan delay of 1 second. Each free induction decay was acquired with 64,000 complex points and an acquisition time of 3.4 seconds. The processed spectra were zero-filled to 64,000 real points, and an exponential apodization function with a line broadening factor (lb) of 0.3 Hz was applied before Fourier transformation.

[0155] SEM

[0156] SEM was performed on a Hitachi SU8030 Scanning Electron Microscope. Samples were coated with 20nm gold. The accelerating voltage was set to 30kV and probe current was set to 75 mA for EDS analysis.

[0157] XPS

[0158] XPS was performed on a ThermoFisher NEXSA-G2 to analyze the surface of the deposition on the Ni electrode. The X-ray source type was monochromated Al K-Alpha. The survey scan was conducted between 0 and 1000 eV with a resolution of 1eV. The survey scan was averaged over 3 sweeps to minimize the noise. Following the survey scans, the elemental scan was conducted between 340 and 360 eV to identify the Ca 2p1 / 2 and 2p3 / 2 peaks with a resolution of 0.1 eV. To minimize the noise, the high resolution elemental scan for Ca was averaged over 10 sweeps.

[0159] XPS confirms the presence of Ca° and Ca+2species on the Ni substrate. Since XPS is a surface analysis technique, negligible signal from Ni was obtained, since the beam was focused on the deposited material.

[0160] Results and Discussion

[0161] Calcium was explored experimentally as a candidate material for N2activation and electrochemical NH3synthesis. Ca was chosen for detailed analysis because it is predicted to be the most reactive, and it has a standard reduction potential of Ca (-2.87 V vs. SHE) close to Li (-3.04 V vs. SHE), suggesting that a similar experimental protocol may succeed. The formation of Ca3N2is favorable due to the spontaneous reaction of N2and Ca, as the free energy of the formation of Ca3N2is -4 eV. It is known that the formation of a bulk nitride can be kinetically slow35 36, but as discussed above, we hypothesize that bulk nitride formation is not necessary for a successful mediator of this process. We hypothesize that Ca-mediated NH3synthesis occurs in a process analogous to Li-mediated NH3synthesis, i.e., based on the following reaction scheme and also shown in Scheme 1 :[Ca electrodeposition] Ca2++ 2e -» Ca(s)(i)[Ca Nitridation] 6Ca(s)+ N2(aq.) 2Ca3N2(s) (ii)[Ca3N2protolysis] Ca3N2(s)+ HX + e” -> Ca3N - NH (iii)[Ca3N2H protolysis] Ca3N - NH* + HX + e Ca3N - NH2+ X' (iv)[Ca3N2H2protolysis] Ca3N - NH2+ HX + e“ Ca3N - NH3+ X' (v)[NH3Desorption] Ca3N - NH3^ Ca3N* + NH3(aq )(vi)[Ca3N Nitridation]

[0162] First, Ca is electrodeposited on a substrate by dissolving Ca salt in a non-aqueous solvent (Calcium Electrodeposition, Eq.(i)) and applying a strongly reducing bias on the cathode. The electrodeposited Ca metal reacts spontaneously with N2(either dissolved or gaseous in a GDE setup) to form Ca3N2(Calcium Nitridation, Eq.(ii)). Following the formation of Ca3N2, a sequence of proton-coupled electron transfer steps (Eqs. (iii)-(v)) forms NH3adsorbed to calcium nitride (Calcium Nitride Protolysis). Finally, NH3desorbs, forming a surface N vacancy on calcium nitride (Eq. (vi)). Given the stability of this surface nitride vacancy, we hypothesize that it is filled by N2, which is subsequently reduced by an associated mechanism in a sequence of CPET reactions, thereby completing the catalytic cycle. Ca metal and nitride can also serve as a catalyst for hydrogen evolution to form H2, an undesired side reaction (Calcium Protolysis). We again note that this proposed mechanism is slightly different than those appearing in some published reports for the analogous Li-mediated NH3synthesis process13 15, which has been hypothesized to involve the concomitant dissolution of Li to form Li+. As discussed above, the precise mechanism is still poorly understood and likely depends strongly on specific details of the experimental protocol in use (e.g., dynamic potential control, electrolyte and salt choice, N2pressure, and others). However, we hypothesize here that concomitant dissolution cannot occur under the strongly reducing conditions typically used, as the corrosion sub-reaction will be endergonic. In this Letter, we provide a proof of concept for the Ca and Mg-mediated NH3synthesis.

[0163] In the lithium system in Example 1 , we observe that increased N2pressure results in improved NH3Faradaic Efficiency (FE). This enhancement extends only to a certain level beyond which there is no improvement in the NH3FE as the system is no longer in an N2mass transport limited regime.10Similarly, there exists an optimal concentration of the proton donor which balances the Li3N protolysis (towards NH3) and Li protolysis (toward HER) steps. Hence, for Ca-mediated NH3synthesis, we decided to operate a reactor at a slightly elevated N2pressure of 6 bar. Ethanol (EtOH, also referred to as HX in Eq. (iii), (iv) and (v)) was used as the proton donor at a concentration of 0.65 M. The reaction was carried out in a modified autoclave setup to withstand high pressures, as shown in Figure 9A. Ni foam was used as the cathode, and Pt was used as the anode in a membrane-less setup, and the electrolyte was stirred at 700 rpm. Tetrahydrofuran (THF) is commonly used as the aprotic solvent to dissolve Li salts in Li-mediated NH3synthesis. One of the challenges in Ca-mediated NH3synthesis is to have a suitable Ca salt that can be dissolved in an aprotic solvent. Calcium salts generally have poor solubility in water and are mostly insoluble in nonaqueous solvents. The Ca salts that are soluble in water generally exist in their hydrated form as they arehygroscopic. Among the tested Ca salts, Ca(CIO4)2 has good solubility in dimethoxy ethane (DME). The water content in freshly prepared pre-electrolytes was measured using a Karl Fischer titrator, yielding an average value of 3.04 ± 0.12%. Experiments were conducted at varying current densities, ranging from 5 mA / cm2up to 45 mA / cm2, as illustrated in 9B. At lower current densities, calcium deposition rate is lower, resulting in a decrease in available Ca sites for nitridation steps, leading to lesser formation of calcium surface nitride. As the current density and, hence, the cell potential increases, the selectivity of calcium nitride protolysis vs. calcium protolysis seems to improve, as interpreted from the increased FE of NH3in Figure 9B. At -15 mA / cm2, the calcium nitridation and calcium nitride protolysis steps seems to reach an optimal balance, resulting in the maximum NH3FE of 50%. At higher current densities, such as -30 mA / cm2and -45 mA / cm2, the cell voltage rapidly increases, which could lead to electrochemical degradation of the solvent and poor formation of SEI. Hence, we observe low NH3FE at higher current densities. An optimal cell voltage of ~4 V is required for stable performance.

[0164] Quantitative analysis of ammonia was conducted using1H NMR spectroscopy, following the previously published protocol that ensures that the measured NH3arises from the electrochemical N2reduction and not from air or other potential contamination sources9. The approach involves the use of15N2isotope as a substrate for electrochemical N2reduction. Since15N isotope is a spin 14 nucleus, the1H NMR spectrum of the ammonia electrogenerated from15N2will give a characteristic doublet at 7.52 ppm with a coupling constant of 180 Hz . This doublet can be readily distinguished from the triplet peak associated with ammonia containing the most abundant14N isotope (spin 1 nucleus), which appears at 6.89 ppm with a coupling constant of 54 Hz. The resulting spectra for the post-electrolyte samples at different current densities are depicted in Figure 9C. Isotope labeling experiments were conducted at - 15 mA / cm2, as this condition yielded the highest FE and ammonia current density. The isotope-labeled experiments were quantified using1H-NMR, as depicted in Figure 9D, resulting in an FE of 50 ± 0.2% at -15 mA / cm2. Rigorous control experiments were performed to ensure that NH3is produced by the electrochemical reduction of N2 and not from adverse contaminants. To depict the accumulation of ammonia over time, two different experiments were performed in a batch system. One experiment ran for 2 hours at -15 mA / cm2, while the other ran for 1 .5 hours at -15 mA / cm2. The isotope-labeled experiments were quantified using1H-NMR, as depicted in 9D, resulting in an FE of 50 ± 0.2 % at -15 mA / cm2. In the isotope labeling experiment conducted for 1.5 hours at -15 mA / cm2, an NH3FE of 38% was obtained, with an ammonia current density of 5.6 mA / cm2. Open circuit control experiments were performed for both14N2and15N2, where no potential was applied, and the solutions were kept on a stir plate for 2 hours. NMR spectra of the pre-electrolyte and post-electrolyte samples forboth cases showed no ammonia in the post-electrolyte samples. Additionally, another control experiment was conducted using argon (Ar) to pressurize the reactor instead of N2. The reactor was pressurized to 6 bar with Ar, and the reaction was carried out at -15 mA / cm2for 2 hours. Post-electrolyte analysis using1H NMR showed no ammonia peaks. Finally, an electrolyte sample was prepared and left open in the fume hood to ensure that ammonia was not coming from atmospheric contaminants, but was instead being electrochemically synthesized. After 2 hours in the fume hood,1H NMR analysis showed no ammonia peaks.

[0165] The characterization of the post-electrolysis catalyst was conducted using Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDS) and X-ray Photoelectron Spectroscopy (XPS) on the post-reaction sample, as depicted in Figures 10A- 10D. The analysis was performed ex-situ just to confirm the deposition of Ca. In-situ analysis to understand the interface would be interesting but it is beyond the scope of the current work.

[0166] References for Example 2(1) MacFarlane, D. R.; Cherepanov, P. V.; Choi, J.; Suryanto, B. H.; Hodgetts, R. Y.; Bakker, J. M.; Vallana, F. M. F.; Simonov, A. N. A roadmap to the ammonia economy. Joule 2020, 4 (6), 1186-1205.(2) Erisman, J. W.; Sutton, M. A.; Galloway, J.; Klimont, Z.; Winiwarter, W. How a century of ammonia synthesis changed the world. Nature geoscience 2008, 1 (10), 636-639.(3) Christensen, C. H.; Johannessen, T.; S rensen, R. Z.; N rskov, J. K. Towards an ammonia-mediated hydrogen economy? Catalysis Today 2006, 111 (1-2), 140-144.(4) Shipman, M. A.; Symes, M. D. Recent progress towards the electrosynthesis of ammonia from sustainable resources. Catalysis Today 2017, 286, 57-68.(5) Fu, X.; Zhang, J.; Kang, Y. Recent advances and challenges of electrochemical ammonia synthesis. Chem Catalysis 2022, 2022, 2 (10), 2590-2613. DOI: https: / / doi.Org / 10.1016 / j.checat.2022.09.001(6) Qing, G.; Ghazfar, R.; Jackowski, S. T.; Habibzadeh, F.; Ashtiani, M. M.; Chen, C.-P.; Smith, M. R., Ill; Hamann, T. W. Recent Advances and Challenges of Electrocatalytic N2 Reduction to Ammonia. Chemical Reviews 2020, 120 (12), 5437-5516. DOI: 10.1021 / acs.chemrev.9b00659.(7) Du, H.-L; Chatti, M.; Hodgetts, R. Y.; Cherepanov, P. V.; Nguyen, C. K.; Matuszek, K.; MacFarlane, D. R.; Simonov, A. N. Electroreduction of nitrogen with almost 100% current-to- ammonia efficiency. Nature 2022, 609 (7928), 722-727.(8) Choi, J.; Du, H.-L; Nguyen, C. K.; Suryanto, B. H.; Simonov, A. N.; MacFarlane, D. R. Electroreduction of nitrates, nitrites, and gaseous nitrogen oxides: a potential source of ammonia in dinitrogen reduction studies. ACS Energy Letters 2020, 5 (6), 2095-2097.(9) Andersen, S. Z.; Colic, V.; Yang, S.; Schwalbe, J. A.; Nielander, A. C.; McEnaney, J. M.; Enemark-Rasmussen, K.; Baker, J. G.; Singh, A. R.; Rohr, B. A. A rigorous electrochemical ammonia synthesis protocol with quantitative isotope measurements. Nature 2019, 570 (7762), 504-508.(10) Andersen, S. Z.; Statt, M. J.; Bukas, V. J.; Shapel, S. G.; Pedersen, J. B.; Krempl, K.; Saccoccio, M.; Chakraborty, D.; Kibsgaard, J.; Vesborg, P. C. Increasing stability, efficiency, and fundamental understanding of lithium-mediated electrochemical nitrogen reduction. Energy & Environmental Science 2020, 13 (1 1), 4291-4300.(1 1) Kani, N. C.; Goyal, I.; Gauthier, J. A.; Shields, W.; Shields, M.; Singh, M. R. Pathway toward Scalable Energy-Efficient Li-Mediated Ammonia Synthesis. ACS Appl Mater Interfaces 2024, 16 (13), 16203-16212. DOI: 10.1021 / acsami.3c19499 From NLM.(12) Sazinas, R.; Li, K.; Andersen, S. Z.; Saccoccio, M.; Li, S.; Pedersen, J. B.; Kibsgaard, J.; Vesborg, P. C.; Chakraborty, D.; Chorkendorff, I. Oxygen-Enhanced Chemical Stability of Lithium-Mediated Electrochemical Ammonia Synthesis. The Journal of Physical Chemistry Letters 2022, 13 (20), 4605-461 1 .(13) Lazouski, N.; Schiffer, Z. J.; Williams, K.; Manthiram, K. Understanding continuous lithium-mediated electrochemical nitrogen reduction. Joule 2019, 3 (4), 1127-1139.(14) Lazouski, N.; Steinberg, K. J.; Gala, M. L.; Krishnamurthy, D.; Viswanathan, V.; Manthiram, K. Proton donors induce a differential transport effect for selectivity toward ammonia in lithium-mediated nitrogen reduction. ACS Catalysis 2022, 12 (9), 5197-5208.(15) Lazouski, N.; Chung, M.; Williams, K.; Gala, M. L; Manthiram, K. Non-aqueous gas diffusion electrodes for rapid ammonia synthesis from nitrogen and water-splitting-derived hydrogen. Nature Catalysis 2020, 3 (5), 463-469.(16) Sazinas, R.; Andersen, S. Z.; Li, K.; Saccoccio, M.; Krempl, K.; Pedersen, J. B.; Kibsgaard, J.; Vesborg, P. C. K.; Chakraborty, D.; Chorkendorff, I. Towards understanding of electrolyte degradation in lithium-mediated non-aqueous electrochemical ammonia synthesis with gas chromatography-mass spectrometry. RSC advances 2021 , 11 (50), 31487-31498.(17) Li, K.; Shapel, S. G.; Hochfilzer, D.; Pedersen, J. B.; Krempl, K.; Andersen, S. Z.; Sazinas, R.; Saccoccio, M.; Li, S.; Chakraborty, D. Increasing Current Density of Li-Mediated Ammonia Synthesis with High Surface Area Copper Electrodes. ACS Energy Letters 2021 , 7 (1), 36-41.(18) Li, K.; Andersen, S. Z.; Statt, M. J.; Saccoccio, M.; Bukas, V. J.; Krempl, K.; Sazinas, R.; Pedersen, J. B.; Shadravan, V.; Zhou, Y. Enhancement of lithium-mediated ammonia synthesis by addition of oxygen. Science 2021 , 374 (6575), 1593-1597.(19) Li, S.; Zhou, Y.; Li, K.; Saccoccio, M.; Sazinas, R.; Andersen, S. Z.; Pedersen, J. B.; Fu, X.; Shadravan, V.; Chakraborty, D. Electrosynthesis of ammonia with high selectivity and high rates via engineering of the solid-electrolyte interphase. Joule 2022, 6 (9), 2083-2101.(20) Fu, X.; Pedersen, J. B.; Zhou, Y.; Saccoccio, M.; Li, S.; Sazinas, R.; Li, K.; Andersen, S. Z.; Xu, A.; Deissler, N. H. Continuous-flow electrosynthesis of ammonia by nitrogen reduction and hydrogen oxidation. Science 2023, 379 (6633), 707-712.(21) Cherepanov, P. V.; Krebsz, M.; Hodgetts, R. Y.; Simonov, A. N.; MacFarlane, D. R. Understanding the factors determining the faradaic efficiency and rate of the lithium redox- mediated N2 reduction to ammonia. The Journal of Physical Chemistry C 2021, 125 (21), 11402-11410.(22) Rakov, D.; Hasanpoor, M.; Baskin, A.; Lawson, J. W.; Chen, F.; Cherepanov, P. V.; Simonov, A. N.; Howlett, P. C.; Forsyth, M. Stable and Efficient Lithium Metal Anode Cycling through Understanding the Effects of Electrolyte Composition and Electrode Preconditioning. Chemistry of Materials 2021, 34 (1), 165-177.(23) Suryanto, B. H.; Matuszek, K.; Choi, J.; Hodgetts, R. Y.; Du, H.-L.; Bakker, J. M.; Kang, C. S.; Cherepanov, P. V.; Simonov, A. N.; MacFarlane, D. R. Nitrogen reduction to ammonia at high efficiency and rates based on a phosphonium proton shuttle. Science 2021, 372 (6547), 1187-1191.(24) Blair, S. J.; Doucet, M.; Niemann, V. A.; Stone, K. H.; Kreider, M. E.; Browning, J. F.; Halbert, C. E.; Wang, H.; Benedek, P.; McShane, E. J.; et al. Combined, time-resolved, in situ neutron reflectometry and X-ray diffraction analysis of dynamic SEI formation during electrochemical N2 reduction. Energy & Environmental Science 2023, 16 (8), 3391-3406, 10.1039 / D2EE03694K. DOI: 10.1039 / D2EE03694K.(25) Montoya, J. H.; Tsai, C.; Vojvodic, A.; Narskov, J. K. The Challenge of Electrochemical Ammonia Synthesis: A New Perspective on the Role of Nitrogen Scaling Relations. ChemSusChem 2015, 8 (13), 2180-2186. DOI: https: / / doi.org / 10.1002 / cssc.201500322.(26) Tort, R.; Bagger, A.; Westhead, O.; Kondo, Y.; Khobnya, A.; Winiwarter, A.; Davies, B. J.V.; Walsh, A.; Katayama, Y.; Yamada, Y.; et al. Searching for the Rules of Electrochemical Nitrogen Fixation. ACS Catalysis 2023, 13 (22), 14513-14522. DOI:10.1021 / acscatal.3c03951.(27) Singh, A. R. A Theoretical Approach Toward Sustainable Ammonia Synthesis; Stanford University, 2019.(28) Choksi, T. S.; Roling, L. T.; Streibel, V.; Abild-Pedersen, F. Predicting adsorption properties of catalytic descriptors on bimetallic nanoalloys with site-specific precision. The journal of Physical Chemistry Letters 2019, 10 (8), 1852-1859.(29) Roling, L. T.; Choksi, T. S.; Abild-Pedersen, F. A coordination-based model for transition metal alloy nanoparticles. Nanoscale 2019, 11 (10), 4438-4452.(30) Roling, L. T.; Abild-Pedersen, F. Structure-Sensitive Scaling Relations: Adsorption Energies from Surface Site Stability. ChemCatChem 2018, 10 (7), 1643-1650.(31) Calcium as a Mediator for the Electrochemical Synthesis of NH3. ChemRxiv. (accessed).(32) Fu, X.; Niemann, V. A.; Zhou, Y.; Li, S.; Zhang, K.; Pedersen, J. B.; Saccoccio, M.; Andersen, S. Z.; Enemark-Rasmussen, K.; Benedek, P.; et al. Calcium-mediated nitrogen reduction for electrochemical ammonia synthesis. 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In Handbook of Preparative Inorganic Chemistry 2nd Ed, Vol. Vol. 1. ; 1963; pp p. 940-941..

[0167] Example 3 - Magnesium System

[0168] Materials and Methods

[0169] Electrochemical Experiments

[0170] The magnesium mediated electrochemical synthesis of ammonia was conducted using a specialized 2-electrode setup, comprising a single-compartment glass cell within a custom- designed autoclave placed in an explosion-proof enclosure. The electrolyte utilized was asolution of 1 M magnesium perchlorate dissolved in dimethyl formamide (DMF), with an addition of 0.065 M ethanol (EtOH), pre-saturated with nitrogen (N2) for 10 minutes at a flow rate of 200 standard cubic centimeters per minute (seem). Nickel (Ni) foam was employed as the cathode, with fresh Ni foam used for each run. Before the reaction, the Ni foam was cleansed with ethanol to eliminate water contamination, followed by drying in an oven at 80°C for 20 minutes. A planar platinum (Pt) anode was utilized, cleaned after each reaction by sonication in acetone and ethanol, followed by oven drying at 80°C for 20 minutes. The Ni foam and Pt anode were electrically connected using copper (Cu) wire and Cu tape. The glass cell and magnetic stirring bar were dried at 80°C for 1 hour before assembly. The distance between the anode and cathode was approximately 1 cm, with a cathode surface area of 4 cm2facing the anode. Before electrochemical experiments, the autoclave was purged with N2 from a gas cylinder 10 times to remove oxygen and other contaminants, and then pressurized to a desired pressure of 6 bars. Electrochemical experiments were carried out using a potentiostat (Biologic SP 300) with stirring at 700 rpm. Chronopotentiometry was conducted following the switching current strategy proposed by Anderson et al., with a current density of -5 mA / cm2for 2 minutes, followed by 0 mA / cm2for 2 minutes, depending on the need to change, stabilize, or adjust the working electrode potential. During the working cycle various current densities ranging from -5 mA / cm2up to -45 mA / cm2were tested. All experiments were conducted at room temperature.

[0171] Colorimetric quantification of products

[0172] NH3 was quantified by the Indophenol blue method using a standard additions method to account for the changes in appearance and properties of the electrolyte solution after the electrochemical reaction. In this method, post electrolyte sample is diluted 100-400 folds with 0.1 M H2SO4 to bring the concentration to the detectable limits. After dilution the ammonia present will be in the form of ammonium ion. Subsequently, the solution is centrifuged twice for 15 minutes each to obtain a clear solution. Calibration solutions are prepared using 0.01 M NH4CI as the stock solution and further diluting it to make 100-500 pM NH4CI calibration solutions in H2SO4. The blank solution for prepared by adding, 500uL of H2SO4, 500 pL of Phenol nitroprusside and 500 pL of alkaline sodium hypochlorite. 6 sample vials were each filled with 400 pL aliquots of the centrifugated diluted post electrolyte solution and 100 pL of internal aqueous standard solutions (0.1 M H2SO4, 100 pM-500 pM NH4CI in 0.1 M H2SO4) along with 500 pL of Phenol nitroprusside followed by 500 pL of alkaline sodium hypochlorite. The mixtures were incubated in dark for 30 minutes at ambient temperature. The sample changes color from colorless to blue. The sample was scanned for absorbance as a function of wavelengths from 400 to 800 nm using a visible spectrometer (Genesys 30 VisibleSpectrometef). The maximum absorbance was observed at 632 nm and hence 632 nm was chosen to measure absorbances to quantify NH3. Absorbances of all the six solutions at 632 nm is plotted linearly against concentration. The ratio of the intercept and the slope is the moles of ammonia in the test sample. After calculating the moles, concentration and partial current density is determined.

[0173] NMR

[0174] For the NMR analyses conducted on the Bruker Neo 600 MHz system with QCI-F cryoprobe, dimethylsulfoxide-d6 (DMSO-d6) was utilized as the deuterated solvent. For the preparation of the sample, 1 mL of post-electrolyte was mixed with 1 mL of 0.1 M H2SO4to convert all the NH3 into ammonium sulphate. The mixture was thoroughly mixed and the solvent was then evaporated using a rotary evaporator. Once the remaining solvent was reduced to 1 / 5 of its original volume, waterwas added to bring it back up to the original volume. The resulting solution was thoroughly mixed and centrifuged. Subsequently, 570 pL of the solution was added to the NMR tube, along with 30 L of 10 mM acetone prepared in DMSO- d6, for further analysis. Subsequently, the resulting solution was transferred into an NMR tube for testing. The measurements were performed using a Bruker spectrometer equipped with a cryoprobe. The data presented herein represent the accumulation of 16 scans. To suppress the water resonance, the excitation sculpting method employing a 3-ms 180° shaped pulse centered at 4.612 ppm was employed. The perfect-echo variant was selected to minimize J- modulation for samples analyzed at 500 MHz. A total of 1 ,024 transient scans were recorded with an interscan delay of 1 second. Each free induction decay was acquired with 64,000 complex points and an acquisition time of 3.4 seconds. The processed spectra were zero- filled to 64,000 real points, and an exponential apodization function with a line broadening factor (lb) of 0.3 Hz was applied before Fourier transformation. MNova software was used to analyze the NMR data. The signal-to-noise ratio was utilized for both calibration samples and product samples.

[0175] SEM

[0176] SEM was performed on a Hitachi SU8030 Scanning Electron Microscope. Samples were coated with 20nm Platinum.

[0177] XPS

[0178] XPS was performed on a ThermoFisher NEXSA-G2 to analyze the surface of the deposition on the Ni electrode. The X-ray source type was monochromated Al K-Alpha. The survey scan was conducted between 0 and 1000 eV with a resolution of 1eV. The survey scan was averaged over 3 sweeps to minimize the noise. Following the survey scans, the elementalscan was conducted between 55 and 47 eV to identify the Mg 2p and 131 1 and 1296 eV for Mg 1 s peaks with a resolution of O.leV. To minimize the noise, the high resolution elemental scan for Mg was averaged over 10 sweeps.

[0179] XPS confirms the presence of Mg on the Ni substrate. Since XPS is a surface analysis technique, negligible signal from Ni was obtained, since the beam was focused on the deposited material. However, when the beam is focused on bare Ni foam, the Ni peaks were clearly observed.

[0180] A high amount of carbon observed in both cases suggests that the organic electrolyte was decomposed and was deposited on the Ni foam.

[0181] Results and Discussion

[0182] Based on the results in Examples 1 and 2, magnesium was investigated driven by its even lower plating potential (-2.37 V vs. SHE), making it a more efficient option. We hypothesize that Mg-mediated NH3synthesis occurs in a process analogous to Li and Ca mediated NH3synthesis.

[0183] In line with the results in Examples 1 and 2, for Mg-mediated NH3synthesis, we chose to operate the reactor at a N2 pressure of 6 bar. Ethanol (EtOH) was designated as the proton donor, with a concentration of 0.065 M. To accommodate the high pressures required for the process, we implemented a modified autoclave setup. Within our experimental framework, Ni foam was employed as the cathode, complemented by Pt serving as the anode in a membrane less configuration. Maintaining a stirring rate of 700 rpm ensured effective mixing of the electrolyte throughout the reaction. However, we encountered challenges with the solubility of magnesium salts in various solvents which were tested for LiMAS and CaMAS, which proved to be quite limited and difficult to establish. We found that Magnesium perchlorate exhibited better solubility, particularly in DMF and propylene carbonate, facilitating our experimental progress. The propylene carbonate system exhibited higher viscosity, leading to increased resistance and, consequently, reduced efficiency. Furthermore, no ammonia was detected in the propylene carbonate system.

[0184] The electrolyte comprised of 1 M Magnesium perchlorate dissolved in DMF along with 0.065M EtOH. We implemented a current switching strategy, with 1 minutes of working time and 1 minutes of resting time to stabilize the Solid Electrolyte Interphase (SEI). The duration of the resting time varied and was adjusted as needed to maintain system stability, resulting in variable total runtimes for each current density. The resting current density was maintained at 0 mA / cm2 (i.e., at open circuit voltage), while different working current densities were applied as shown in Figures 11A-1 1 B. The switching time is crucial for stabilizing cell voltageby limiting the growth of the Solid Electrolyte Interphase (SEI) and preserving its stability. During the working time, when negative current density is applied, magnesium (Mg) deposition primarily occurs. The rate-limiting steps during this phase include Mg nitridation, Mg3N protolysis, and Mg protolysis11. Conversely, during the resting time, when the current density is zero, Mg deposition ceases, and thermochemical nitridation and protolysis steps become dominant. The extent of Mg nitridation is contingent upon the N2 pressure, while Mg3N protolysis and Mg protolysis are influenced by the concentration of the proton donor, ethanol (EtOH). The formation of the passivating SEI requires time to develop, approximately one hour under the reported conditions, and selectively permits the passage of Mg+2 ions, N2, H2, NH3, and H+11. We theorize that the SEI enhances cell voltage stability by hindering the reduction of DMF at the electrode. Switching times also impact mass transport. It has been experimentally observed that during resting time, the open circuit potential should be maintained close to -0.5V. Deviations favor H2 formation through Mg protolysis. Thus, optimizing the resting time is crucial. Stirring speed also affects Mg3N and Mg protolysis and may vary based on the size and type of stirrer employed11. If the open circuit potential deviates significantly from 0.5V, adjustments can be made to restore it. Lowering the resting time, working time, and stirring speed can bring the potential back to 0.5V if it drops below this value. Conversely, increasing the resting time, reducing the working time, and enhancing the stirring speed can restore the potential to 0.5V if it rises above this threshold. Based on our observations, we found that 1 minute of working time and 1 up to 5 minutes of resting time are optimal for stabilizing the SEI depending on the total current density.

[0185] When the current density (CD) increases, the NH3 CDs rise while the NH3 FEs remain constant at an average value of -27% as shown in Error! Reference source not found.. The loss in FE could be attributed to excess Mg deposition, electrolyte breakdown, and corrosion. Figures 12A-12B demonstrate that the production rates of NH3 and H2 increase proportionally with rising current density. This suggests that the ratio of Mg protolysis to Mg3N2protolysis rates is independent of potential, or perhaps both protolysis steps are barrierless at high potentials. Therefore, the potential-independent behavior could be due to a fixed fractional coverage of Mg3N2on the cathode since protolysis is a thermochemical step. The coverage of Mg3N2is balanced by nitridation and protolysis reactions, which are directly related to N2pressure and proton donor, respectively. Figures 13A-13B present a comparison of Faradaic Efficiency (FE) and ammonia Current Density (CD) for LIMAS, CaMAS, and MgMAS, all tested in our lab. As anticipated, LIMAS demonstrates superior ammonia CD, benefiting from extensive research that has optimized conditions and thoroughly studied all relevant parameters. CaMAS also shows significant potential with good ammonia selectivity, althoughfurther work is needed to enhance both the ammonia CD and FE. MgMAS, while promising, still requires in-depth research to achieve comparable FEs and CDs.

[0186] Open circuit control experiments were conducted where no potential was applied, and the solutions were stirred for 2 hours with 6 bars of N2pressure. NMR spectra of both preelectrolyte and post-electrolyte samples showed no ammonia in the post-electrolyte samples, as shown in Figure 14B. Additionally, another control experiment was performed using argon (Ar) to pressurize the reactor instead of N2. The reactor was pressurized to 6 bar with Ar, and the reaction was carried out at -15 mA / cm2for4 hours. Post-electrolyte analysis using1H NMR showed no ammonia peaks, as shown in 14A. Finally, an electrolyte sample was prepared and left open in the fume hood to ensure that ammonia was not originating from atmospheric contaminants but was instead being electrochemically synthesized. After 2 hours in the fume hood,1H NMR analysis showed no ammonia peaks as depicted in 14B.

[0187] Quantitative analysis of ammonia was conducted using1H NMR spectroscopy, following a previously published protocol that ensures the measured NH3arises from electrochemical N2reduction and not from air or other potential contamination sources9. The triplet peak associated with ammonia containing the most abundant14N2isotope (spin 1 nucleus) appears at 6.83 ppm with a coupling constant of 96 Hz as shown in Error! Reference source not found.. The resulting spectra for the post-electrolyte samples at different current densities are depicted in 15A. Rigorous control experiments were performed to ensure that NH3is produced by the electrochemical reduction of N2and not from contaminants. UV-vis spectrometry was also used for quantification through the indophenol method. Both UV-vis and NMR were benchmarked and produced consistent results. The method of additions was employed to accurately determine the ammonia.

[0188] The post-electrolysis catalyst was characterized using Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDS) and X-ray Photoelectron Spectroscopy (XPS) on the post-reaction sample, as illustrated in Error! Reference source not found.Figures 16A-16D. This ex-situ analysis was conducted to confirm the deposition of Mg.

[0189] References for Example 3(1) Goyal, I. K., Nishithan; Olusegun, Samuel; Chinnabattigalla, Sreenivasulu; Bhawnani, Rajan; Glusac, Ksenija; Singh, Aayush; Gauthier, Joseph; Singh, Meenesh. Metal Nitride as a Mediator for the Electrochemical Synthesis of NH3. ACS Energy letters 2024. DOI: acsenergylett.4c01455.(2) Iriawan, H.; Andersen, S. Z.; Zhang, X.; Comer, B. M.; Barrio, J.; Chen, P.; Medford, A. J.; Stephens, I. E. L; Chorkendorff, I.; Shao-Horn, Y. Methods for nitrogen activation by reduction and oxidation. Nature Reviews Methods Primers 2021 , 1 (1), 56. DOI: 10.1038 / S43586-021- 00053-y.(3) Smil, V. Enriching the Earth: Fritz Haber, Carl Bosch, and the Transformation of World Food Production-, The MIT Press, 2000. DOI: 10.7551 / mitpress / 2767.001.0001.(4) Chen, S.; Qi, G.; Yin, R.; Liu, Q.; Feng, L.; Feng, X.; Hu, G.; Luo, J.; Liu, X.; Liu, W. Electrocatalytic nitrate-to-ammonia conversion on CoO / CuO nanoarrays using Zn-nitrate batteries. Nanoscale 2023, 15 (48), 19577-19585, 10.1039 / D3NR05254K. DOI: 10.1039 / D3NR05254K.(5) Lu, G.; Gao, S.; Liu, Q.; Zhang, S.; Luo, J.; Liu, X. Design of material regulatory mechanism for electrocatalytic converting NO / NO3- to NH3 progress. Natural Sciences 2023, 3 (3), e20220047. DOI: https: / / doi.org / 10.1002 / ntls.20220047.(6) Yang, M.; Meng, G.; Li, H.; Wei, T.; Liu, Q.; He, J.; Feng, L.; Sun, X.; Liu, X. Bifunctional bimetallic oxide nanowires for high-efficiency electrosynthesis of 2,5-furandicarboxylic acid and ammonia. Journal of Colloid and Interface Science 2023, 652, 155-163. DOI: https: / / doi.org / 10.1016 / jjcis.2023.08.079.(7) Kani, N. C.; Nguyen, N. H. L.; Markel, K.; Bhawnani, R. R.; Shindel, B.; Sharma, K.; Kim, S.; Dravid, V. P.; Berry, V.; Gauthier, J. A.; et al. Electrochemical Reduction of Nitrates on CoO Nanoclusters-Functionalized Graphene with Highest Mass Activity and Nearly 100% Selectivity to Ammonia. Advanced Energy Materials 2023, 13 (17), 2204236. DOI: https: / / doi.org / 10.1002 / aenm.202204236.(8) Kani, N. C.; Prajapati, A.; Singh, M. R. Sustainable Routes for Photo-Electrochemical Synthesis of Ammonia Using Various Nitrogen Precursors. ACS ES&T Engineering 2022, 2 (6), 1080-1087. DOI: 10.1021 / acsestengg.1c00484.(9) Andersen, S. Z.; Colic, V.; Yang, S.; Schwalbe, J. A.; Nielander, A. C.; McEnaney, J. M.; Enemark-Rasmussen, K.; Baker, J. G.; Singh, A. R.; Rohr, B. A.; et al. A rigorous electrochemical ammonia synthesis protocol with quantitative isotope measurements. Nature 2019, 570 (7762), 504-508. DOI: 10.1038 / s41586-019-1260-x From NLM.(10) Du, H.-L.; Chatti, M.; Hodgetts, R. Y.; Cherepanov, P. V.; Nguyen, C. K.; Matuszek, K.; MacFarlane, D. R.; Simonov, A. N. Electroreduction of nitrogen with almost 100% current-to- ammonia efficiency. Nature 2022, 609 (7928), 722-727. DOI: 10.1038 / s41586-022-05108-y.(11) Kani, N. C.; Goyal, I.; Gauthier, J. A.; Shields, W.; Shields, M.; Singh, M. R. Pathway toward Scalable Energy-Efficient Li-Mediated Ammonia Synthesis. ACS Appl Mater Interfaces 2024, 76 (13), 16203-16212. DOI: 10.1021 / acsami.3c19499 From NLM.(12) Kim, K.; Lee, S. J.; Kim, D. Y.; Yoo, C. Y.; Choi, J. W.; Kim, J. N.; Woo, Y.; Yoon, H. C.; Han, J. I. Electrochemical Synthesis of Ammonia from Water and Nitrogen: A Lithium- Mediated Approach Using Lithium-Ion Conducting Glass Ceramics. ChemSusChem 2018, 77 (1), 120-124, Article. DOI: 10.1002 / cssc.201701975 Scopus.(13) Lazouski, N.; Chung, M.; Williams, K.; Gala, M. L; Manthiram, K. Non-aqueous gas diffusion electrodes for rapid ammonia synthesis from nitrogen and water-splitting-derived hydrogen. Nature Catalysis 2020, 3 (5), 463-469. DOI: 10.1038 / s41929-020-0455-8.(14) Schwalbe, J. A.; Statt, M. J.; Chosy, C.; Singh, A. R.; Rohr, B. A.; Nielander, A. C.; Andersen, S. Z.; McEnaney, J. M.; Baker, J. G.; Jaramillo, T. F.; et al. A Combined Theory- Experiment Analysis of the Surface Species in Lithium-Mediated NH3 Electrosynthesis. ChemElectroChem 2020, 7 (7), 1542-1549. DOI: https: / / doi.org / 10.1002 / celc.201902124.(15) Tsuneto, A.; Kudo, A.; Sakata, T. Lithium-mediated electrochemical reduction of high pressure N2 to NH3. Journal of Electroanalytical Chemistry 1994, 367 (1), 183-188. DOI: https: / / doi.Org / 10.1016 / 0022-0728(93)03025-K.(16) Krebsz, M.; Hodgetts, R. Y.; Johnston, S.; Nguyen, C. K.; Hora, Y.; MacFarlane, D. R.; Simonov, A. N. Reduction of dinitrogen to ammonium through a magnesium-based electrochemical process at close-to-ambient temperature. Energy & Environmental Science 2024, 77 (13), 4481-4487, 10.1039 / D4EE01090F. DOI: 10.1039 / D4EE01090F.(17) Andersen, S. Z.; Statt, M. J.; Bukas, V. J.; Shapel, S. G.; Pedersen, J. B.; Krempl, K.; Saccoccio, M.; Chakraborty, D.; Kibsgaard, J.; Vesborg, P. C. K.; et al. 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[0190] It should be emphasized that the above-described embodiments of the present disclosure are merely possible examples of implementations set forth for a clear understanding of the principles of the disclosure. Many variations and modifications may be made to the above-described embodiment(s) without departing substantially from the spirit and principles of the disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.

Claims

CLAIMS1. A method for producing ammonia, the method comprising introducing nitrogen gas at a pressure greater than ambient pressure into an electrochemical cell, wherein the electrochemical cell comprises(a) a reaction chamber;(b) an anode and cathode positioned within the reaction chamber;(c) a voltage source connecting the cathode to the anode;(d) an ionically conductive membrane positioned between the cathode and the anode; and(e) an electrolyte composition comprising (i) an organic solvent, (ii) a proton donor, and (iii) a salt of lithium, calcium, magnesium, strontium, yttrium, scandium, zirconium, or any combination, wherein the electrolyte composition is positioned between the anode and cathode, wherein a current is applied to the electrochemical cell to convert nitrogen to ammonia.

2. The method of claim 1 , wherein the pressure of the nitrogen gas in the electrochemical cell is from 1 bar to 100 bar.

3. The method of claim 1 or 2, wherein the salt of lithium, calcium, magnesium, strontium, yttrium, scandium, or zirconium is a tetrafluoroborate salt, a hexafluorophosphate a hexafluoroarsenate salt, a perchlorate salt, a triflate salt, a bisoxalato borate salt, a difluorooxalato borate salt, trifluorosulfonylimide salt, or a halide salt.

4. The method of any one of claims 1-3, wherein the salt of lithium, calcium, magnesium, strontium, yttrium, scandium, or zirconium has a concentration of about 0.5 M to about 4.0 M in the electrolyte composition.

5. The method of any one of claims 1-4, wherein the proton donor comprises an alcohol.

6. The method of any one of claims 1-4, wherein the proton donor comprises an alkyl alcohol, an aryl alcohol, or a cycloalkyl alcohol.

7. The method of any one of claims 1-4, wherein the proton donor comprises a C1 to C7 alkyl alcohol.

8. The method of any one of claims 1-4, wherein the proton donor comprises ethanol.

9. The method of any one of claims 1-8, wherein the proton donor has a concentration of about 0.01 M to about 0.40 M in the electrolyte composition.

10. The method of any one of claims 1-9, wherein the organic solvent comprises an ether.

11. The method of claim 10, wherein the ether comprises dimethyl ether, diethyl ether, diglyme, or triglyme,12. The method of any one of claims 1-9, wherein the organic solvent comprises a fluorinated solvent.

13. The method of claim 12, wherein the fluorinated solvent comprises a perfluoro alkane or fluorobenzene.

14. The method of any one of claims 1-9, wherein the organic solvent comprises tetrahydrofuran, dimethylsulfoxide, dimethoxyethane, N,N-dimethylformamide, or any combination thereof.

15. The method of any of claims 1-14, wherein the current applied to the electrochemical cell has a current density from about -200 mA / cm2to about 2,500 mA / cm2.

16. The method of claim 1 , wherein the electrolyte composition comprises a calcium salt and the proton donor comprises ethanol, and the pressure of nitrogen in the electrochemical cell is from 1 bar to 10 bar.

17. The method of claim 16, wherein the calcium salt is calcium perchlorate.

18. The method of claim 16 or 17, wherein the organic solvent comprises dimethoxymethane.

19. The method of any of claims 16-18, wherein the current applied to the electrochemical cell has a current density from about -5 mA / cm2to about -45 mA / cm2.

20. The method of claim 1 , wherein the electrolyte composition comprises a magnesium salt, the proton donor comprises ethanol, and the pressure of nitrogen in the electrochemical cell is from 1 bar to 10 bar.

21. The method of claim 20, wherein the magnesium salt is magnesium perchlorate.

22. The method of claim 20 or 21 , wherein organic solvent comprises N,N-dimethylformamide.

23. The method of any of claims 20-22, wherein the current applied to the electrochemical cell has a current density from about -40 mA / cm2to about -20 mA / cm2.

24. The method of claim 1 , wherein the electrolyte composition comprises a lithium salt, the proton donor comprises ethanol, and the pressure of nitrogen in the electrochemical cell is from 1 bar to 100 bar.

25. The method of claim 24, wherein the lithium salt is lithium perchlorate.

26. The method of claim 24 or 25, wherein organic solvent comprises tetrahydrofuran.

27. The method of any of claims 24-26, wherein the current applied to the electrochemical cell has a current density from about -150 mA / cm2to about 600 mA / cm2.

28. The method of any one of claims 16-27, wherein ethanol has a concentration of about 0.03 M to about 0.08 M in the electrolyte composition.

29. The method of any of claims 1-28, wherein the anode comprises nickel, platinum, tungsten, a metal alloy, a metal oxide, or graphite.

30. The method of any of claims 1-29, wherein the cathode comprises steel, nickel, copper, titanium, molybdenum, or graphite.

31. The method of any of claims 1-30, wherein the ionically conductive membrane comprises a glass or polymer.

32. The method of claim 31, wherein the polymer comprises polyethylene or polypropylene.

33. The method of any of claims 1-32, wherein the temperature in the reaction chamber is from about 25 °C to about 30 °C.

34. The method of any of claims 1-33, wherein (i) the current is applied for a first period of time, (ii), the current is turned off for second period of time, and (iii) the current is applied for a third period of time.

35. The method of any of claims 1-34, wherein the electrolyte composition is stirred in the electrochemical cell.

36. The method of any of claims 1-35, wherein the reaction chamber comprises stainless steel.

37. The method of any of claims 1-36, wherein the reaction chamber comprises a first port for introducing the electrolyte composition into the reaction chamber and a second port for introducing nitrogen into the reaction chamber.

38. The method of any of claims 1-37, wherein the reaction chamber comprises a pressure gauge for measuring the pressure within the reaction chamber.

39. The method of any of claims 1-38, wherein after ammonia is produced, removing the ammonia from the reaction chamber.

40. The method of any of claims 1-39, wherein after ammonia is produced, (i) removing electrolyte composition comprising ammonia from the reaction chamber and (ii) removing ammonia from the electrolyte composition.

41. The method of claim 40, wherein after removal of ammonia, the electrolyte composition is regenerated and introduced into the reaction chamber.

42. The method of any of claims 1-41 , wherein the method is performed continuously to produce ammonia.

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